Elementary Botany

I. General Form and Arrangement of Leaves.

746. Influence of foliage leaves on the form of the stem.—The marked effect which foliage has upon the aspect of the plant or upon the landscape is evident to all observers. Perhaps it is usual to look upon the stem as having been developed for the display of the foliage without taking into account the possibility that the foliage may have a great influence upon the form or habit of the stem. It is very evident, however, that the foliage exercises a great influence on the form of the stem. For example, as trees increase in age and size, the development of branches on the interior ceases and some of those already formed die, since the dense foliage on the periphery of the trees cuts off the necessary light stimulus. The tree, therefore, possesses fewer branches and a more open interior. In the forest also, the dense foliage above makes possible the shapely, clean timber trunks. Note certain trees where by accident, or by design, the terminal foliage-bearing branches have been removed that foliage-bearing branches may arise in the interior of the tree system.

Without foliage leaves the stems of green plants would develop a very different habit from what they do. This development could take place in three different directions under the influence of light: (1) The light stimulus would induce profuse branching, so that there would be many small branches. (2) The stem would develop fewer branches, but they would be flattened. (3) Massive trunks with but few or no branches. In fact, all these forms are found in certain green stems which do not bear leaves. An example of the first is found in asparagus with its numerous crowded slender branches. But such forms in our climate are rare, since foliage leaves are more efficient. The second and third forms are found among cacti, which usually grow in dry regions under conditions which would be fatal to ordinary thin foliage leaves.

747. Relation of foliage leaves to the stem.—In the study of the position of the leaves on the stem we observe two important modes of distribution: (1) the distribution along the individual stem or branch which bears them, usually classed under the head of Phyllotaxy; (2) the distribution of the leaves with reference to the plant as a whole.

748. Phyllotaxy, or arrangement of leaves.—In examining buds on the winter shoots of woody plants, we cannot fail to be impressed with some peculiarities in the arrangement of these members on the stem of the plant.

In the horse-chestnut, as we have already observed, the leaves are in pairs, each one of the pair standing opposite its partner, while the pair just below or above stand across the stem at right angles to the position of the former pair. In other cases (the common bed-straw) the leaves are in whorls, that is, several stand at the same level on the axis, distributed around the stem. By far the larger number of plants have their leaves arranged alternately. A simple example of alternate leaves is presented by the elm, where the leaves stand successively on alternate sides of the stem, so that the distance from one leaf to the next, as one would measure around the stem, is exactly one half the distance around the stem. This arrangement is one half, or the angle of divergence of one leaf from the next is one half. In the case of the sedges the angle of divergence is less, that is one-third.

By far the larger number of those plants which have the alternate arrangement have the leaves set at an angle of divergence represented by the fraction two fifths. Other angles of divergence have been discovered, and much stress has been laid on what is termed a law in the growth of the stem with reference to the position which the leaves occupy. Singularly by adding together the numerators and denominators of the last two fractions gives the next higher angle of divergence. Example:

and so on. There are, however, numerous exceptions to this regular arrangement, which have caused some to question the importance of any theory like that of the “spiral theory” of growth propounded by Goethe and others of his time.

749. Adaptation in leaf arrangement.—As a result, however, of one arrangement or another we see a beautiful adaptation of the plant parts to environment, or the influence which environment, especially light, has had on the arrangement of the leaves and branches of the plant. Access to light and air are of the greatest importance to green plants, and one cannot fail to be profoundly impressed with the workings of the natural laws in obedience to which the great variety of plants have worked out this adaptation in manifold ways.

750. Distribution of leaves with reference to the entire plant.—In this case, as in the former, we recognize that it is primarily a light relation. As the plant becomes larger and more branched the lower and inner leaves disappear. The trees and shrubs have by far the larger number of leaves on the periphery of the branch system. A comparison of different kinds of trees in this respect shows, however, that there is great variation. Trees with dense foliage (elm, Norway maple, etc.) present numerous leaves on the periphery which admit but little light to the interior where leaves are very few or wanting. The sugar maple and red maple do not cast such a dense shade and there are more leaves in the interior. This is more marked in the silver maple, and still more so in the locust (Gleditschia tricanthos).

751. Color of foliage leaves.—The great majority of foliage leaves are green in color. This we have learned (Chapter VII) is due to the presence of a green pigment, chlorophyll, in the chloroplastids thickly scattered in the cells of the leaf. We have also learned that in the great majority of cases, the light stimulus is necessary for the production of chlorophyll green. There are many foliage leaves which possess other colors, as red (Rosa rubrifolia), purple (the purple barberry, hazel, beech, birch, etc.), yellow (the golden oak, elder, etc.); while many others have more or less deep tints of pink, red, purple, yellow, when young. All of these leaves, however, possess chlorophyll in addition to red, yellow, purple or other pigment. These other pigments are sometimes developed in great quantity in the cell-sap. They obscure the chlorophyll from view, but do not interfere seriously with the action of light and the function of chlorophyll, and perhaps in some cases serve as a screen to protect the protoplast.

752. Autumn colors.—Foliage leaves of many trees display in the autumn gorgeous colors. These colors are principally shades of red or yellow, and sometimes purple. The autumn color is more marked in some trees than in others. In the red maple, the red and scarlet oak, the sourwood, etc., red predominates, though sometimes yellow may be present with the red in a single leaf. Sugar maples, poplars, hickories, etc., are principally yellow in autumn. The sweet gum has a rich variety of color-red, purple, maroon, yellow; sometimes all these colors are present on the same tree.

The red and purple colors are found suffused in the cell-sap of certain cells in the leaf much as we have found it in the cells of the red beet. The yellow color is chiefly due to the disappearance and degeneration of the chlorophyll while the leaf is in a moribund state. A similar phenomenon is seen in the yellowing of crops when the soil becomes too wet, or in the blanching of grass when covered with a board, or of celery as the earth is ridged up over the leaves in late summer and autumn. A number of different theories have been advanced to explain autumn coloring, i.e., the appearance of the red coloring matter. It has been attributed to the approach of cold weather, and this has likely led to the erroneous belief on the part of some that it is caused by frost. It very often precedes frost. Some have attributed it to the action of the more oblique light rays during autumn, and still others to the diminishing water-supply with the approach of cool weather. The question is one which has not met as yet with a satisfactory solution, and is certainly a very obscure one. It is likely that the low temperature or the declining activities of the leaf affect certain organic substances in the leaf and give rise to the red color, and it is quite certain that in some years the display is more brilliant than in others. The color is more striking in some regions than in others and the different soil, as well as climate, has been supposed to have some influence. The North American forests are noted for the brilliant display of autumnal color. This is perhaps due to some extent to the great variety or number of species which display color. It would seem that there is some specific as well as individual peculiarities in certain trees. Some individuals, for example, exhibit brilliant colors every autumn, while others near of the same species are more subdued.

It has been shown by experiment that when sunlight passes through red colors the temperature is slightly increased, and it has been suggested that this may be of protection to the living substance which has ceased working and is in danger of injury from cold. There does not seem to be much ground for this suggestion, however. It certainly could not protect the protoplasm of the leaf at night when the cold is more intense, and during the day would only aggravate matters by supplying an increased amount of heat, since extremes of heat and cold in alternation are more harmful to plant life than uniform cold. Especially would this be the case in alpine climates where the alternation of heat and cold between day and night is extreme, and brilliancy of the colors of alpine plants is well known. It seems more reasonable to suppose that the red color acts as a screen, as the chlorophyll is disappearing, to protect from the injurious action of light, certain organic substances which are to be transferred back from the leaf to the stem for winter storage. So in the case of many stems in the spring or early summer when the young leaves often have a reddish color, it is likely that it acts as a screen to protect the living substance from the strong light at that season of the year until the chlorophyll screen, which is weak in young leaves, becomes darker in color and more effective, when the red color often disappears.

753. Function of foliage leaves.—In general the function of the foliage leaf as an organ of the plant is fivefold (see Chapters IV, VII, VIII, XI), (1) that of carbon dioxide assimilation or photosynthesis, (2) that of transpiration, (3) that of the synthesis of other organic compounds, (4) that of respiration, and (5) that of assimilation proper, or the making of new living substance. While none of these functions are solely carried on in the leaf, it is the chief seat of the first three of these processes, its form, position, and structure being especially adapted to the purpose. Assimilation proper, as well as respiration, probably take place equally in all growing or active parts.

754. Parts of the leaf.—All foliage leaves possess a blade or lamina, so called because of its expanded and thin character. The blade is the essential part. Many leaves, however, are provided with a stalk or petiole by which the blade is held out at a greater or lesser distance from the stem. Leaves with no petiole are sessile, the blade is attached by one end directly on the stem. In some cases the base of the blade is wrapped partly around the stem, or in others it extends entirely around the stem and is perfoliate. Besides, many leaves have short appendages, termed stipules, attached usually on opposite sides of the petiole at its junction with the stem. In some species of magnolia the stipules are so large that each one envelops the entire portion of the bud which has not yet opened. Many leaves possess outgrowths in the form of hairs, scales, etc. (See leaf protection.)

755. Simple leaves.—Simple leaves are those in which the blade is plane along the edge, not divided. The edge may be entire or indented (serrate) to a slight extent as in the elm. The form of the simple leaf varies greatly but is usually constant for a given species, or it may vary in shape in the same species on different parts of the plant. Some of the terms applied to the outline of the leaf are ovate, oval, elliptical, lanceolate, linear, needle-like, etc., but it is idle for one to waste time on matters of minute detail in form until it becomes necessary for those in the future who pursue taxonomic work. It is evident that a simple leaf, except those of minute size, possesses advantages over a divided leaf in the amount of surface it exposes to the light. But in other respects it is at a disadvantage, especially as it increases in size, since it casts a deeper shade and does not admit of such a free circulation of air. It will be found, however, in our study of the relation of leaves to light and air that the balance between the leaf and its environment is obtained in the relation of the leaves to each other.

756. Venation of leaves.—A very prominent character of the leaf is its “venation.” This is indicated by the presence of numerous “veins,” indicated usually by narrow depressed lines on the upper surface, and by more or less distinct elevated lines on the under surface. There are two general types: (1) In the corn, Smilacina, Solomon’s seal, etc., the veins extend lengthwise of the leaf and are nearly parallel. Such leaves are said to be parallel-veined. It is generally, though not always, a character of monocotyledenous plants. (2) In the elm, rose, hawthorn, maple, oak, etc., the veins are not all parallel. The larger ones either diverge from the base of the blade (palmate leaf, maple), or the midvein extends through the middle line of the leaf, while other prominent ones branch off from this and extend, nearly parallel, toward the edge of the leaf (pinnate venation). The smaller intermediate veins which are also very distinct extend irregularly and branch and anastomose in such a fashion as to give the figure of a net with very fine meshes. These are netted-veined leaves. These are characteristic of most of the dicotyledenous plants. It is evident from what has been said of the examples cited that there are two types of netted-veined leaves, the palmate and pinnate.

Note. As we have already learned in Chapter V the veins contain the vascular bundles of the leaf. Through them the water and food solutions are distributed to all parts of the leaf, and the return current of food material elaborated in the leaf moves back through the bast portion into the shoot. The veins also possess a small amount of mechanical tissue. This forms the framework of the leaf and aids in giving rigidity to the leaf and in holding it in the expanded position. The mechanical tissue in the framework alone could not support the leaf. Turgescence of the mesophyll is needed in addition.

757. Cut or lobed leaves.—In many leaves, the indentations on the margin are few and deep. Such leaves present several lobes the proportionate size of which is dependent upon the depth of the indentation or “incision.” Several of the maples, oaks, birches, the poison-ivy, thistles, the dandelion, etc., have lobed leaves. Where the indentation reaches to or very near the midrib the leaf is said to be cut. A study of various leaves will show all gradations from simple leaves with plane edges to those which are cut or divided, as in compound leaves, and the lobes are often variously indented.

758. Divided, or compound leaves.—The rose, sumac, elder, hickory, walnut, locust, pea, clover, American creeper, etc., are examples of divided or compound leaves. The former are pinnately compound, and the latter are palmately compound. The leaf of the honey-locust is twice pinnately compound or bipinnate, and some are three times pinnately compound.[44] It is evident that compound leaves are only extreme forms of lobed or cut leaves and that the form of all bears a definite relation to the primary venation. There has been a reduction of mesophyll and of the area of smaller venation.

759. These forms of leaves probably have some definite significance. It is not quite clear why they should have developed as they have; though it is possible to explain several important relations of these forms to their environment. (1) The reduction of the surface of the leaf, with the retention of the firmer portions, allows freer movement of the air and affords the leaf greater protection from injury during violent winds, just as the finely dissected leaves of some water plants are less liable to injury from movement of the more dense medium in which they live. It is possible that here we may have an explanation of one of the factors involved in this reduction of leaf surface. (2) In trees with compound leaves, like the hickory, walnut, locust, ailanthus, etc., the midvein, and in the case of the Kentucky coffee-tree (Gymnocladus) the primary lateral veins also, serve in place of terminal branches of the stem. By the increase in the outline of the leaf and the reduction of its surface between the larger veins, the tree has attained the same leaf development that it would were the larger veins replaced by stems bearing simple leaves. The tree as it is, however, has the advantage of being able to cast off for the winter period a layer of what otherwise would have been a portion of the stem system, to retain which through the winter would use more energy than with the present reduced stem system, and the stouter stem is less liable to dry out. In the case of herbaceous plants, in the case of plants like most of the ferns where the stem is on the underground rootstock (Pteris), or a very short erect stem, as in the Christmas fern, the leaf replaces the aerial stem, and the division (or branching, as it is sometimes styled) of the leaf corresponds to the branching of the stem. This is more marked in the gigantic exotics like Cibotium regale, and in the tree ferns which have quite tall trunks, the massive compound leaves replace branches. In the palms and cycads are similar examples. Those who choose to observe can doubtless find many examples close at hand. (3) While divided leaves have probably not been evolved in response to the light relation, still their relation in this respect is an important one, since if the leaf with its present size were entire, it would cast too dense a shade on other leaves below.

760. General structure of the leaf.—The general structure of the leaf has been already studied (see Chapters IV, V, VII). It is only necessary to recall the main points. The upper and lower surfaces of the leaf are provided with a layer of cells usually devoid of chlorophyll. The mesophyll of the leaf consists usually of a layer of palisade cells beneath the epidermis, and the remainder consists of loose parenchyma with large intercellular spaces. Through the mesophyll course the “veins,” or fibrovascular strands, consisting of the xylem and phloem portions and serving as conduits for water, salts, and foodstuffs. In the epidermis are the stomata, each one protected by the two guard cells. The guard cells as well as the mesophyll contain chlorophyll. The stomata and the communicating intercellular spaces furnish the avenues for the ingress and egress of gases, and for the escape of water vapor.

761. Protection of leaves.—There are many modifications of the general plan of structure in different leaves, many of them being adaptations for the protection of the leaf under adverse or trying conditions. Many leaves are also capable of assuming certain positions which afford them protection. The discussion of this subject may be presented under two general heads: Protective modifications; protective positions.

II. Protective Modification of Leaves.

762. General directions in which these modifications have taken place.—The usual type of foliage leaf selected is that of deciduous trees or shrubs or of our common herbs. Such a leaf is usually greatly expanded and thin in order to present as great a surface as possible in comparison with its mass, since the kind of work which the leaf has to do can be more effectually carried on when it possesses this form. This form of leaf is best adapted for work in regions where there is a medium amount of moisture such as exists in the temperate zones. But since there are very great variations in the climatic and soil conditions of these regions, and even greater changes in desert and arctic regions, the type of leaf described is unsuited for all. Its own life would be endangered, and it would also endanger the life of the plant. Modifications have therefore taken place to meet these conditions, or at least those plants whose leaves have become modified in those directions which are suited to the surrounding conditions have been able to persist. Excessive cold or heat, drought, winds, intense light, rain, etc., are some of the conditions which endanger leaves. The protective modifications of leaves may be grouped under four general heads: (1) Structural adaptations; (2) Protective covering; (3) Reduction of surface; (4) Elimination of the leaf through the complete assumption of the leaf function by the stem.

763. (1) Structural adaptations.—The general structure of the leaf presents certain features which are protective. The palisade layer of cells found usually beneath the upper epidermis forms a compact layer of long cells which not only acts as a light screen cutting off a certain amount of the light, since too intense light would be harmful; it also aids in lessening the loss of water from the upper surface, where radiation is greater. The stomata are usually on the under side of aerial leaves, and the mechanism which closes them when the leaf is losing too much water is protective. As a protection against intense light the number of palisade layers is sometimes increased or the cells of this layer are narrow and long. This is often beautifully shown when comparing leaves of the same plant grown in strong light with those grown in the shade. The compass plant (Lactuca scariola) affords an interesting example. The leaves grown in the light are usually vertical, so that the light reaches both sides. Such leaves often have all of the mesophyll organized into palisade cells (fig. 435), while leaves grown in the deep shade may have no palisade cells.

764. (2) Protective covering.—Epidermis and cuticle.—The walls of the epidermal cells are much thickened in some plants. Sometimes this thickening occurs in the outer wall, or both walls may be thickened. Variation in this respect as well as the extent of the thickening occur in different plants and are often correlated with the extremes of conditions which they serve to meet. The cuticle, a waxy exudation from the thick wall of the epidermis of many leaves, also serves as a protection against too great loss of water, or against the leaf becoming saturated with water during rains. The cabbage, carnation, etc., have a well-developed cuticle. The effect of the cuticle in shedding water can be nicely shown by spraying water on a cabbage leaf or by immersing it in water. Sunken stomata also retard the loss of water vapor.

Covers of hair or scales.—In many leaves certain of the cells of the epidermis grow out into the form of hairs or scales of various forms, and they serve a variety of purposes. When the hairs form a felt-like covering as in the common mullein, some antennarias, etc., they lessen the loss of water vapor because the air-currents close to the surface of the leaf are retarded. Spines (see the thistles, etc.) also afford a protection against certain animals.

765. (3) Reduction of surface.—Reduction of leaf surface is brought about in a variety of ways. There are two general modes: (1st) Reduction of surface along with reduction of mass; (2d) Reduction of surface inversely as the mass. Examples of the first mode are seen in the dissected leaves of many aquatic plants. In this finely dissected condition the mass of the leaf substance is much reduced as well as the leaf surface, but the leaf is less liable to be injured by movement of the water. In addition it has already been pointed out that lobed and divided aerial leaves are much less liable to injury from violent movements of the air, than if a leaf with the same general outline were entire. The needle leaves of the conifers are also examples, and they show as well structural provisions for protection in the thick, hard cell-walls of the epidermis. To offset the reduced surface there are numerous crowded leaves. Reduction of surface inversely as the mass, i.e., the mass of the leaf may not be reduced at all, or it may be more or less increased. In other words, there is less leaf surface in proportion to the mass of leaf substance. It is probable in many cases, example: the crowded, overlapping small scale leaves of the juniper, arbor-vitæ, cypress, cassiope, pyxidanthera, etc., that there has been a reduction in the size of the leaf, and at the same time an increase in thickness. This with the crowding together of the leaves and their thick cell-walls greatly lessens the radiation of moisture and heat, thus protecting the leaves both in dry and cold weather. The succulents, like “live-forever,” have a small amount of surface in proportion to the mass of the leaf. In the yucca, though the leaves are often large, they are very thick and expose a comparatively small amount of surface to the dry air and intense sunlight of the desert regions. The epidermal covering is also hard and thick. In addition, such leaves, as well as those of many succulents, are so thick they provide water storage sufficient for the plants, which radiate so slowly from their surface.

766. (4) Elimination of the leaf.—Perhaps the most striking illustration of the reduction of leaf surface is in those cases where the leaf is either completely eliminated as in certain euphorbias, or in certain of the cacti where the leaves are thought to be reduced to spines. Whether the cactus spine belongs to the leaf series or not, the leaf as an organ for assimilation and transpiration has been completely eliminated and the same is true in the phylloclades. The leaf function has been assumed by the stem. The stem in this case contains all the chlorophyll; is bulky, and provides water storage.

III. Protective Positions.

767. In many cases the leaves are arranged either in relation to the stem, or to each other, or to the ground, in such a way as to give protection from too great radiation of heat or moisture. In the examples already cited the imbricated leaves of cassiope, pyxidanthera, juniper, etc., come also under this head. In the junipers the leaves spread out in the summer, while in the winter they are closely overlapped. An interesting example of protective position is to be seen in the case of the leaves of the white pine. During quite cold winter weather the needles are appressed to the stem, and sometimes the trees present a striking appearance in contrast with the spreading position of the needles in summer. On windy days in winter, the needles turn with the wind and become rigid in that position so that they remain in a horizontal position for some time, often until the wind dies down, or until milder weather. The following day, should there be a cold strong wind from the opposite direction, the needles again assume a leeward direction. In quiet weather appressed to the stem and in the form of a brush there is less radiation of heat than if they diverged. In strong winds by turning in the leeward direction the wind is not driven between the needle bases and scales. Some plants, especially many of those in arctic and alpine regions, have very short stems and the leaves are developed near the ground, or the rock. Lying close on the ground they do not feel the full force of the drying winds, there is less radiation from them, and the radiation of heat from the ground protects them. Many plants exhibit movement in response to certain stimuli which place them in a position for protection. Some of these examples have been discussed under the head of irritability (see Chapter XIII). The night position of leaves and cotyledons presented by many plants, but especially by many of the Leguminosæ, is brought about by the removal of the light stimulus at evening. In many leaves, when the light influence is removed, the influence of growth turns the leaves downward, or the cotyledons of some plants upward. In this vertical position of the leaf-blade there is less radiation of heat during the cool night. The most striking cases of protection movements are seen in the sensitive plant. As we have seen, the leaves of mimosa close in a vertical position at midday if the light and heat are too strong. Excessive transpiration is thus prevented. At night the vertical position prevents excessive radiation of heat. The vertical or profile position of the leaves of the compass plant already referred to not only lessens transpiration, but the intense heat and light of the midday sun is avoided. This profile position is characteristic of certain plants in the dry regions of Australia, and the topmost leaves of tropical forests.

IV. Relation of Leaves to Light.

768. It is very obvious from our study of the function of the foliage leaf that its most important relation to environment is that which brings it in touch with light and air. It is necessary that light penetrate the leaf tissue that the gases of the air and plant may readily diffuse and that water vapor may pass out of the leaf. The thin expanded leaf-blade is the most economical and efficient organ for leaf work. We have seen that leaves respond to fight stimulus in such a way as to bring their upper sides usually to face the source of fight, at right angles to it or nearly so (heliotropism, see Chapter XIII). How fully this is brought about depends on the kind of plant, as well as on other elements of the environment, for as we have seen in our study of leaf protection there is danger to some plants in any region, and to other plants in certain regions that the intense light and heat may harm the protoplast, or the chlorophyll, or both.

The statement that leaves usually face the light at right angles is to be taken as a generalized one. The source of the strongest illumination varies on different days and again at different times of the day. On cloudy days the zenith is the source of strongest illumination. The horizontal position of a leaf, where there are no intercepting lateral or superior objects would receive its strongest light rays perpendicular to its surface. The fact is, however, that leaves on the same stem, because of taller or shorter adjacent stems, are so situated that the rays of greatest illuminating power are directed at some angle between the zenith and horizon. Many leaves, then, which may have their upper sides facing the general source of strongest illumination, do not necessarily face the sun, and they are thus protected from possible injury from intense light and heat because the direct rays of sunlight are for the most part oblique. This does not apply, of course, to those leaves which “follow the sun” during the day. Their specific constitution is such that intense illumination is beneficial.

The leaf is adjusted as well as may be in different species of varying constitution, and under different conditions, to a certain balance in its relation to the factors concerned. The problem then is to interpret from this point of view the positions and grouping of leaves. Because of the specific constitution of different plants, and because of a great variety of conditions in the environment, we see that it is a more or less complex question.

769. Day and night positions contrasted.—In many plants the day and night positions of the leaves are different. At night the leaves assume a position more or less vertical, known as the profile position. This is generally regarded as a protective position, since during the cool of the night the radiation of heat is less than if the leaf were in a vertical position. In many of these plants, however, the leaves in assuming the night position become closely appressed which would also lessen the radiation. This peculiarity of leaves is largely possessed by the members of the family Leguminoseæ (clovers, peas, beans, etc.), and by the sensitive plants.[45] But it is also shared by some other plants as well (oxalis, for example). The leaves of these plants are usually provided with a mechanism which enables them to execute these movements with ease. There is a cushion (pulvinus) of tissue at the base of the petiole, and in the case of compound leaves, at the base of the pinnæ and pinnules which undergoes changes in turgor in its cells. The collapsing of the cells by loss of water into the intercellular spaces causes the leaf to droop. When the cells regain their turgor by the absorption of the water from the intercellular spaces the leaf is raised to the horizontal, or day position. The light stimulus induces turgor of the pulvinus, the disappearance of the stimulus is accompanied by a loss of turgor. It is a remarkable fact that in some sensitive plants, intense light stimuli are alarm signals which result in the same movement as if the light stimulus were entirely removed. As we know also contact or pressure stimulus, or jarring produces the same result in “sensitive” plants like mimosa, some species of rubus, etc. In many plants there is no well-developed pulvinus, and yet the leaves show similar movements in assuming the day and night positions. Examples are seen in the sunflower, and in the cotyledons of many plants. A little observation will enable any one interested to discover some of these plants.[46] In these cases the night position is due to epinastic growth, and while this influence is not removed during the day the light stimulus overcomes it and the leaf is raised to the day position.

770. Leaves which rotate with the sun.—During the growth period the leaves of the sunflower as well as the growing end of the stem respond readily to the direct sunlight. The response is so complete that during sunny days the leaves toward the growing end of the stem are drawn close together in the form of a rosette and the entire rosette as well as the end of the stem are turned so that they face the sun directly. In the morning under the stimulus of the rising sun the rosette is formed and faces the east. All through the day, if the sun continues to shine, the leaves follow it, and at sundown the rosette faces squarely the western horizon. For a week or more the young sunflower head will also face the sun directly and follow it all day as surely as the rosette of leaves. At length, a little while before the flowers in the head blossom, the head ceases to turn, but the rosette of leaves and the stem also, to some extent, continue to turn with the sun. When the leaves become mature they also cease to turn. This is well shown in all three photographs (figs. 438-439). The lower leaves on the stem being older have assumed the fixed horizontal position usually characteristic of the plant with cylindrical habit.

It is not true, as is commonly supposed, that the fully opened sunflower head turns with the sun. But I have observed young heads four or five inches in diameter rotate with the sun all day. This is because the growing end of the stem as well as the young head responds to the light stimulus. So there is some truth as well as a great deal of fiction in the popular belief that the sunflower head follows the sun. The young head will follow the sun all day even if all the leaves are cut off, and the growing stem will also if all the leaves as well as the flower head are cut away. Young seedlings will also turn even if the cotyledons and plumule are cut off.

This phenomenon of the rotation of leaves with the sun is much more general than one would infer, as may be seen from a little careful observation of rapidly growing plants on bright sunny days. In Alabama I have observed beautiful rosettes of Cassia marilandica rotate with the sun all day. The peculiarity is very striking in the cotton plant, especially when the rows extend north and south. In the forenoon or afternoon it is most striking as the entire row shows the leaves tilted up facing the sun. There are many of our weeds and common flowers of field and garden which show this rotation of the leaves. Some of these form rotating rosettes; while in others the leaves rotate independently as in the sweet clover.

771. Fixed position of old leaves.—In many of the cases cited in the preceding paragraph, the rotation of the leaf only occurs on sunny days. During cloudy days the leaves of the sunflower, for example, are in a nearly horizontal position, or the lower ones may be somewhat oblique, since the stronger illumination on such a plant would be the oblique rays rather than the zenith rays. As the leaves reach maturity also the epinastic growth is equalized by hyponastic growth so that the growth movements bring the leaf to stand in a nearly horizontal position, or that position in which it receives the best illumination. In age, then, many leaves have a fixed position and this corresponds with the position assumed on cloudy days.

772. Position on horizontal stems.—On horizontal stems the leaves have a horizontal position, and if such a stem is stood in an erect position the appearance is very odd. If the leaf arises directly from the horizontal stem, its petiole will be twisted part way around in order to bring the face of the leaf uppermost. It is interesting to observe the different relation of stem, petiole and blade and the amount of twisting as the horizontal stem or vine trails over irregularities in the surface, or climbs over and through other vegetation.

773. Position of leaflets on divided leaves.—An interesting comparison can be made with entire, lobed, divided and dissected leaves. The entire leaf usually lies in one plane, since usually the problem of adjustment is the same for the entire surface. So the lobes of a leaf usually lie all in the same plane as they would if the leaf were entire. We find the same is true usually of the compound leaf. It forms an incomplete mosaic. Some of the pieces having been removed allow much of the light to pass through to leaves beneath. Leaves, especially those of some size rarely lie in a flat plane. Some are more or less depressed. Some curve downward. Compound leaves often curve more or less and the leaflets often droop more or less in a graceful fashion. It is interesting, however, that these far separated leaflets all lie in the same general plane. This is because the area of the leaf, if not too large, makes the problem of position with reference to light much the same as if the leaf were entire. The leaflets or divisions, though separated, are laminate, and they can work more efficiently facing the light. But suppose we extend our observation to the finely dissected capillary leaves of some of the parsley family (Umbelliferæ), or to the upper leaves of the fennel-leaved thoroughwort (Eupatorium fœniculaceum) among the aerial plants, and to Myriophyllum among the aquatic plants. The divisions are thread-like or cylindrical. One side of the leaflet is just as efficient when presented to the light as another. As a result the leaflets are not arranged in the same plane, but stand out in many directions.

Occasionally one finds a divided or compound leaf in such a position that one portion, because of being shaded above, receives the stronger light stimulus from the side, while the other portion is lighted from above. If this relation continues throughout the growth-period of the leaf the leaflets of one portion may lie in a different plane from those of the other portion. In such cases, some of the leaflets are permanently twisted to bring them into their proper light relation.

V. Leaf Patterns.

MOSAICS, OR CLOSE PATTERNS.

774. Where the leaves of a plant, or a portion of a plant, are approximate and arranged in the form of a pattern, the leaves fitting together to form a more or less even and continuous surface, such patterns are sometimes termed “mosaics,” since the relation of leaves to one another is roughly like the relation of the pieces of a mosaic. A good illustration of a mosaic is presented by a greenhouse plant Fittonia (fig. 441). The stems are prostrate and the erect branches quite short, but it may have quite a wide system by the spreading of the runners; the branches of such a length that the leaves borne near the tips all fit together forming a broad surface of leaves so closely fitted together often that the stems cannot be seen. The advantage of a mosaic over a separate disposition of leaves at somewhat different levels is that the leaves do not shade one another. Were all the light rays coming down at right angles to the leaves, there would not be any shading of the lower ones, but the oblique rays of light would be cut off from many of the leaves. In the case of a mosaic all the rays of light play upon all the leaves. Some of the mosaics which can be observed are as follows: