472. Movements under Irritation. The most familiar case is that of the Sensitive Plant (Fig. 490). The leaves suddenly take their nocturnal position when roughly touched or when shocked by a jar. The leaflets close in pairs, the four outspread partial petioles come closer together, and the common petiole is depressed. The seat of the movements is at the base of the leaf-stalk and stalklets. Schrankia, a near relative of the Sensitive Plant, acts in the same way, but is slower. These are not anomalous actions, but only extreme manifestations of a faculty more or less common in foliage. In Locust and Honey-Locusts for example, repeated jars will slowly produce similar effects.

473. Leaf-stalks and tendrils are adapted to their uses in climbing by a similar sensitiveness. The coiling of the leaf-stalk is in response to a kind of irritation produced by contact with the supporting body. This may be shown by gentle rubbing or prolonged pressure upon the upper face of the leaf-stalk, which is soon followed by a curvature. Tendrils are still more sensitive to contact or light friction. This causes the free end of the tendril to coil round the support, and the sensitiveness, propagated downward along the tendril, causes that side of it to become less turgescent or the opposite side more so, thus throwing the tendril into coils. This shortening draws the plant up to the support. Tendrils which have not laid hold will at length commonly coil spontaneously, in a simple coil, from the free apex downward. In Sicyos, Echinocystis, and the above mentioned Passion-flowers (471), the tendril is so sensitive, under a high summer temperature, that it will curve and coil promptly after one or two light strokes by the hand.

475. Movements in Flowers are very various. The most remarkable are in some way connected with fertilization (Sect. XIII.). Some occur under irritation: the stamens of Barberry start forward when touched at the base inside: those of many polyandrous flowers (of Sparmannia very strikingly) spread outwardly when lightly brushed: the two lips or lobes of the stigma in Mimulus close after a touch. Some are automatic and are connected with dichogamy (339): the style of Sabbatia and of large-flowered species of Epilobium bends over strongly to one side or turns downward when the blossom opens, but slowly erects itself a day or two later.

476. Extraordinary Movements connected with Capture of Insects. The most striking cases are those of Drosera and Dionæa; for an account of which see "How Plants Behave," and Goodale's "Physiological Botany."

477. The upper face of the leaves of the common species of Drosera, or Sundew, is beset with stout bristles, having a glandular tip. This tip secretes a drop of a clear but very viscid liquid, which glistens like a dew-drop in the sun; whence the popular name. When a fly or other small insect, attracted by the liquid, alights upon the leaf, the viscid drops are so tenacious that they hold it fast. In struggling it only becomes more completely entangled. Now the neighboring bristles, which have not been touched, slowly bend inward from all sides toward the captured insect, and bring their sticky apex against its body, thus increasing the number of bonds. Moreover, the blade of the leaf commonly aids in the capture by becoming concave, its sides or edges turning inward, which brings still more of the gland-tipped bristles into contact with the captive's body. The insect perishes; the clear liquid disappears, apparently by absorption into the tissue of the leaf. It is thought that the absorbed secretion takes with it some of the juices of the insect or the products of its decomposition.

478. Dionæa muscipula, the most remarkable vegetable fly-trap (Fig. 176, 492), is related to the Sundews, and has a more special and active apparatus for fly-catching, formed of the summit of the leaf. The two halves of this rounded body move as if they were hinged upon the midrib; their edges are fringed with spiny but not glandular bristles, which interlock when the organ closes. Upon the face are two or three short and delicate bristles, which are sensitive. They do not themselves move when touched, but they propagate the sensitiveness to the organ itself, causing it to close with a quick movement. In a fresh and vigorous leaf, under a high summer temperature, and when the trap lies widely open, a touch of any one of the minute bristles on the face, by the finger or any extraneous body, springs the trap (so to say), and it closes suddenly; but after an hour or so it opens again. When a fly or other small insect alights on the trap, it closes in the same manner, and so quickly that the intercrossing marginal bristles obstruct the egress of the insect, unless it be a small one and not worth taking. Afterwards and more slowly it completely closes, and presses down upon the prey; then some hidden glands pour out a glairy liquid, which dissolves out the juices of the insect's body; next all is re-absorbed into the plant, and the trap opens to repeat the operation. But the same leaf perhaps never captures more than two or three insects. It ages instead, becomes more rigid and motionless, or decays away.

479. That some few plants should thus take animal food will appear less surprising when it is considered that hosts of plants of the lower grade, known as Fungi, moulds, rusts, ferments, Bacteria, etc., live upon animal or other organized matter, either decaying or living. That plants should execute movements in order to accomplish the ends of their existence is less surprising now when it is known that the living substance of plants and animals is essentially the same; that the beings of both kingdoms partake of a common life, to which, as they rise in the scale, other and higher endowments are successively superadded.

480. Work uses up material and energy in plants as well as in animals. The latter live and work by the consumption and decomposition of that which plants have assimilated into organizable matter through an energy derived from the sun, and which is, so to say, stored up in the assimilated products. In every internal action, as well as in every movement and exertion, some portion of this assimilated matter is transformed and of its stored energy expended. The steam-engine is an organism for converting the sun's radiant energy, stored up by plants in the fuel, into mechanical work. An animal is an engine fed by vegetable fuel in the same or other forms, from the same source, by the decomposition of which it also does mechanical work. The plant is the producer of food and accumulator of solar energy or force. But the plant, like the animal, is a consumer whenever and by so much as it does any work except its great work of assimilation. Every internal change and movement, every transformation, such as that of starch into sugar and of sugar into cell-walls, as well as every movement of parts which becomes externally visible, is done at the expense of a certain amount of its assimilated matter and of its stored energy; that is, by the decomposition or combustion of sugar or some such product into carbonic acid and water, which is given back to the air, just as in the animal it is given back to the air in respiration. So the respiration of plants is as real and as essential as that of animals. But what plants consume or decompose in their life and action is of insignificant amount in comparison with what they compose.

Section XVII. CRYPTOGAMOUS OR FLOWERLESS PLANTS.

482. Linnæus gave to this lower grade of plants the name of Cryptogamia, thereby indicating that their organs answering to stamens and pistils, if they had any, were recondite and unknown. There is no valid reason why this long-familiar name should not be kept up, along with the counterpart one of Phanerogamia (6), although organs analogous to stamens and pistil, or rather to pollen and ovule, have been discovered in all the higher and most of the lower grades of this series of plants. So also the English synonymous name of Flowerless Plants is both good and convenient: for they have not flowers in the proper sense. The essentials of flowers are stamens and pistils, giving rise to seeds, and the essential of a seed is an embryo (8). Cryptogamous or Flowerless plants are propagated by Spores; and a spore is not an embryo-plantlet, but mostly a single plant-cell (399).

483. Vascular Cryptogams, which compose the higher orders of this series of plants, have stems and (usually) leaves, constructed upon the general plan of ordinary plants; that is, they have wood (wood-cells and vessels, 408) in the stem and leaves, in the latter as a frame work of veins. But the lower grades, having only the more elementary cellular structure, are called Cellular Cryptogams. Far the larger number of the former are Ferns: wherefore that class has been called

484. Pteridophyta, Pteridophytes in English form, meaning Fern-plants,—that is, Ferns and their relatives. They are mainly Horsetails, Ferns, Club-Mosses, and various aquatics which have been called Hydropterides, i. e. Water-Ferns.

485. Horsetails, Equisetaceæ, is the name of a family which consists only (among now-living plants) of Equisetum, the botanical name of Horsetail and Scouring Rush. They have hollow stems, with partitions at the nodes; the leaves consist only of a whorl of scales at each node, these coalescent into a sheath: from the axils of these leaf-scales, in many species, branches grow out, which are similar to the stem but on a much smaller scale, close-jointed, and with the tips of the leaves more apparent. At the apex of the stem appears the fructification, as it is called for lack of a better term, in the form of a short spike or head. This consists of a good number of stalked shields, bearing on their inner or under face several wedge-shaped spore-cases. The spore-cases when they ripen open down the inner side and discharge a great number of green spores of a size large enough to be well seen by a hand-glass. The spores are aided in their discharge and dissemination by four club-shaped threads attached to one part of them. These are hygrometric: when moist they are rolled up over the spore; when dry they straighten, and exhibit lively movements, closing over the spore when breathed upon, and unrolling promptly a moment after as they dry. (See Fig. 493-498.)

486. Ferns, or Filices, a most attractive family of plants, are very numerous and varied. In warm and equable climates some rise into forest-trees, with habit of Palms; but most of them are perennial herbs. The wood of a Fern-trunk is very different, however, from that of a palm, or of any exogenous stem either. A section is represented in Fig. 500. The curved plates of wood each terminate upward in a leaf-stalk. The subterranean trunk or stem of any strong-growing herbaceous Fern shows a similar structure. Most Ferns are circinate in the bud; that is, are rolled up in the manner shown in Fig. 197. Uncoiling as they grow, they have some likeness to a crosier.

487. The fructification of Ferns is borne on the back or under side of the leaves. The early botanists thought this such a peculiarity that they always called a Fern-leaf a Frond, and its petiole a Stipe. Usage continues these terms, although they are superfluous. The fruit of Ferns consists of Spore-cases, technically Sporangia, which grow out of the veins of the leaf. Sometimes these are distributed over the whole lower surface of the leaf or frond, or over the whole surface when there are no proper leaf-blades to the frond, but all is reduced to stalks. Commonly the spore-cases occupy only detached spots or lines, each of which is called a Sorus, or in English merely a Fruit-dot. In many Ferns these fruit-dots are naked; in others they are produced under a scale-like bit of membrane, called an Indusium. In Maidenhair-Ferns a little lobe of the leaf is folded back over each fruit-dot, to serve as its shield or indusium. In the true Brake or Bracken (Pteris) the whole edge of the fruit-bearing part of the leaf is folded back over it like a hem.

488. The form and structure of the spore-cases can be made out with a common hand magnifying glass. The commonest kind (shown in Fig. 503) has a stalk formed of a row of jointed cells, and is itself composed of a layer of thin-walled cells, but is incompletely surrounded by a border of thicker-walled cells, forming the Ring. This extends from the stalk up one side of the spore-case, round its summit, descends on the other side, but there gradually vanishes. In ripening and drying the shrinking of the cells of the ring on the outer side causes it to straighten; in doing so it tears the spore-case open on the weaker side and discharges the minute spores that fill it, commonly with a jerk which scatters them to the wind. Another kind of spore-case (Fig. 507) is stalkless, and has its ring-cells forming a kind of cap at the top: at maturity it splits from top to bottom by a regular dehiscence. A third kind is of firm texture and opens across into two valves, like a clam-shell (Fig. 508^a): this kind makes an approach to the next family.

489. The spores germinate on moistened ground. In a conservatory they may be found germinating on a damp wall or on the edges of a well-watered flower-pot. Instead of directly forming a fern-plantlet, the spore grows first into a body which closely resembles a small Liverwort. This is named a Prothallus (Fig. 509): from some point of this a bud appears to originate, which produces the first fern-leaf, soon followed by a second and third, and so the stem and leaves of the plant are set up.

490. Investigation of this prothallus under the microscope resulted in the discovery of a wholly unsuspected kind of fertilization, taking place at this germinating stage of the plant. On the under side of the prothallus two kinds of organs appear (Fig. 510). One may be likened to an open and depressed ovule, with a single cell at bottom answering to nucleus; the other, to an anther; but instead of pollen, it discharges corkscrew-shaped microscopic filaments, which bear some cilia of extreme tenuity, by the rapid vibration of which the filaments move freely over a wet surface. These filaments travel over the surface of the prothallus, and even to other prothalli (for there are natural hybrid Ferns), reach and enter the ovule-like cavities, and fertilize the cell. This thereupon sets up a growth, forms a vegetable bud, and so develops the new plant.

491. An essentially similar process of fertilization has been discovered in the preceding and the following families of Pteridophytes; but it is mostly subterranean and very difficult to observe.

492. Club-Mosses or Lycopodiums. Some of the common kinds, called Ground Pine, are familiar, being largely used for Christmas wreaths and other decoration. They are low evergreens, some creeping, all with considerable wood in their stems: this thickly beset with small leaves. In the axils of some of these leaves, or more commonly, in the axils of peculiar leaves changed into bracts (as in Fig. 511, 512) spore-cases appear, as roundish or kidney-shaped bodies, of firm texture, opening round the top into two valves, and discharging a great quantity of a very fine yellow powder, the spores.

493. The Selaginellas have been separated from Lycopodium, which they much resemble, because they produce two kinds of spores, in separate spore-cases. One kind (Microspores) is just that of Lycopodium; the other consists of only four large spores (Macrospores), in a spore-case which usually breaks in pieces at maturity (Fig. 513-515).

494. The Quillworts, Isoetes (Fig. 516-519), are very unlike Club Mosses in aspect, but have been associated with them. They look more like Rushes, and live in water, or partly out of it. A very short stem, like a corm, bears a cluster of roots underneath; above it is covered by the broad bases of a cluster of awl-shaped or thread-shaped leaves. The spore-cases are immersed in the bases of the leaves. The outer leaf-bases contain numerous macrospores; the inner are filled with innumerable microspores.

495. The Pillworts (Marsilia and Pilularia) are low aquatics, which bear globular or pill-shaped fruit (Sporocarps) on the lower part of their leaf-stalks or on their slender creeping stems. The leaves of the commoner species of Marsilia might be taken for four-leaved Clover. (See Fig. 520.) The sporocarps are usually raised on a short stalk. Within they are divided lengthwise by a partition, and then crosswise by several partitions. These partitions bear numerous delicate sacs or spore-cases of two kinds, intermixed. The larger ones contain each a large spore, or macrospore; the smaller contain numerous microspores, immersed in mucilage. At maturity the fruit bursts or splits open at top, and the two kinds of spores are discharged. The large ones in germination produce a small prothallus; upon which the contents of the microspores act in the same way as in Ferns, and with a similar result.

496. Azolla is a little floating plant, looking like a small Liverwort or Moss. Its branches are covered with minute and scale-shaped leaves. On the under side of the branches are found egg-shaped thin-walled sporocarps of two kinds. The small ones open across and discharge microspores; the larger burst irregularly, and bring to view globose spore-cases, attached to the bottom of the sporocarp by a slender stalk. These delicate spore-cases burst and set free about four macrospores, which are fertilized at germination, in the manner of the Pillworts and Quillworts. (See Fig. 521-526.)

497. Cellular Cryptogams (483) are so called because composed, even in their higher forms, of cellular tissue only, without proper wood-cells or vessels. Many of the lower kinds are mere plates, or ribbons, or simple rows of cells, or even single cells. But their highest orders follow the plan of Ferns and phanerogamous plants in having stem and leaves for their upward growth, and commonly roots, or at least rootlets, to attach them to the soil, or to trunks, or to other bodies on which they grow. Plants of this grade are chiefly Mosses. So as a whole they take the name of

498. Bryophyta, Bryophytes in English form, Bryum being the Greek name of a Moss. These plants are of two principal kinds: true Mosses (Musci, which is their Latin name in the plural); and Hepatic Mosses, or Liverworts (Hepaticæ).

499. Mosses or Musci. The pale Peat-mosses (species of Sphagnum, the principal component of sphagnous bogs) and the strong-growing Hair-cap Moss (Polytrichum) are among the larger and commoner representatives of this numerous family; while Fountain Moss (Fontinalis) in running water sometimes attains the length of a yard or more. On the other hand, some are barely individually distinguishable to the naked eye. Fig. 527 represents a common little Moss, enlarged to about twelve times its natural size; and by its side is part of a leaf, much magnified, showing that it is composed of cellular tissue (parenchyma-cells) only. The leaves of Mosses are always simple, distinct, and sessile on the stem. The fructification is an urn-shaped spore-case, in this as in most cases raised on a slender stalk. The spore-case loosely bears on its summit a thin and pointed cap, like a candle-extinguisher, called a Calyptra. Detaching this, it is found that the spore-case is like a pyxis (376), that is, the top at maturity comes off as a lid (Operculum); and that the interior is filled with a green powder, the spores, which are discharged through the open mouth. In most Mosses there is a fringe of one or two rows of teeth or membrane around this mouth or orifice, the Peristome. When moist the peristome closes hygrometrically over the orifice more or less; when drier the teeth or processes commonly bend outward or recurve; and then the spores more readily escape. In Hair-cap Moss a membrane is stretched quite across the mouth, like a drum-head, retaining the spores until this wears away. See Figures 527-541 for details.

500. Fertilization in Mosses is by the analogues of stamens and pistils, which are hidden in the axils of leaves, or in the cluster of leaves at the end of the stem. The analogue of the anther (Antheridium) is a cellular sac, which in bursting discharges innumerable delicate cells floating in a mucilaginous liquid; each of these bursts and sets free a vibratile self-moving thread. These threads, one or more, reach the orifice of the pistil-shaped body, the Pistillidium, and act upon a particular cell at its base within. This cell in its growth develops into the spore-case and its stalk (when there is any), carrying on its summit the wall of the pistillidium, which becomes the calyptra.

501. Liverworts or Hepatic Mosses (Hepaticæ) in some kinds resemble true Mosses, having distinct stem and leaves, although their leaves occasionally run together; while in others there is no distinction of stem and leaf, but the whole plant is a leaf-like body, which produces rootlets on the lower face and its fructification on the upper. Those of the moss-like kind (sometimes called Scale-Mosses) have their tender spore-cases splitting into four valves; and with their spores are intermixed some slender spiral and very hygrometric threads (called Elaters) which are thought to aid in the dispersion of the spores. (Fig. 542-544.)

502. Marchantia, the commonest and largest of the true Liverworts, forms large green plates or fronds on damp and shady ground, and sends up from some part of the upper face a stout stalk, ending in a several-lobed umbrella-shaped body, under the lobes of which hang several thin-walled spore-cases, which burst open and discharge spores and elaters. Riccia natans (Fig. 545) consists of wedge-shaped or heart-shaped fronds, which float free in pools of still water. The under face bears copious rootlets; in the substance of the upper face are the spore-cases, their pointed tips merely projecting: there they burst open, and discharge their spores. These are comparatively few and large, and are in fours; so they are very like the macrospores of Pillworts or Quillworts.

503. Thallophyta, or Thallophytes in English form. This is the name for the lower class of Cellular Cryptogams,—plants in which there is no marked distinction into root, stem, and leaves. Roots in any proper sense they never have, as organs for absorbing, although some of the larger Seaweeds (such as the Sea Colander, Fig. 553) have them as holdfasts. Instead of axis and foliage, there is a stratum of frond, in such plants commonly called a Thallus (by a strained use of a Greek and Latin word which means a green shoot or bough), which may have any kind of form, leaf-like, stem-like, branchy, extended to a flat plate, or gathered into a sphere, or drawn out into threads, or reduced to a single row of cells, or even reduced to single cells. Indeed, Thallophytes are so multifarious, so numerous in kinds, so protean in their stages and transformations, so recondite in their fructification, and many so microscopic in size, either of the plant itself or its essential organs, that they have to be elaborately described in separate books and made subjects of special study.

505. Lichens live in the air, that is, on the ground, or on rocks, trunks, walls, and the like, and grow when moistened by rains. They assimilate air, water, and some earthy matter, just as do ordinary plants. Algæ, or Seaweeds, live in water, and live the same kind of life as do ordinary plants. Fungi, whatever medium they inhabit, live as animals do, upon organic matter,—upon what other plants have assimilated, or upon the products of their decay. True as these general distinctions are, it is no less true that these orders run together in their lowest forms; and that Algæ and Fungi may be traced down into forms so low and simple that no clear line can be drawn between them; and even into forms of which it is uncertain whether they should be called plants or animals. It is as well to say that they are not high enough in rank to be distinctively either the one or the other. On the other hand there is a peculiar group of plants, which in simplicity of composition resemble the simpler Algæ, while in fructification and in the arrangements of their simple cells into stem and branches they seem to be of a higher order, viz.:—

506. Characeæ. These are aquatic herbs, of considerable size, abounding in ponds. The simpler kinds (Nitella) have the stem formed of a single row of tubular cells, and at the nodes, or junction of the cells, a whorl of similar branches. Chara (Fig. 550-552) is the same, except that the cells which make up the stem and the principal branches are strengthened by a coating of many smaller tubular cells, applied to the surface of the main or central cell. The fructification consists of a globular sporocarp of considerable size, which is spirally enwrapped by tubular cells twisted around it: by the side of this is a smaller and globular antheridium. The latter breaks up into eight shield-shaped pieces, with an internal stalk, and bearing long and ribbon shaped filaments, which consist of a row of delicate cells, each of which discharges a free-moving microscopic thread (the analogue of the pollen or pollen-tube), nearly in the manner of Ferns and Mosses. One of these threads reaches and fertilizes a cell at the apex of the nucleus or solid body of the sporocarp. This subsequently germinates and forms a new individual.

507. Algæ or Seaweeds. The proper Seaweeds may be studied by the aid of Professor Farlow's "Marine Algæ of New England;" the fresh-water species, by Prof. H. C. Woods's "Fresh-water Algæ of North America," a larger and less accessible volume. A few common forms are here very briefly mentioned and illustrated, to give an idea of the family. But they are of almost endless diversity.

508. The common Rockweed (Fucus vesiculosus, Fig. 554, abounding between high and low water mark on the coast), the rarer Sea Colander (Agarum Turneri, Fig. 553), and Laminaria, of which the larger forms are called Devil's Aprons, are good representatives of the olive green or brownish Seaweeds. They are attached either by a disk-like base or by root-like holdfasts to the rocks or stones on which they grow.

509. The hollow and inflated places in the Fucus vesiculosus or Rockweed (Fig. 554) are air-bladders for buoyancy. The fructification forms in the substance of the tips of the frond: the rough dots mark the places where the conceptacles open. The spores and the fertilizing cells are in different plants. Sections of the two kinds of conceptacles are given in Fig. 555 and 556. The contents of the conceptacles are discharged through a small orifice which in each figure is at the margin of the page. The large spores are formed eight together in a mother-cell. The minute motile filaments of the antheridia fertilize the large spores after injection into the water: and then the latter promptly acquire a cell-wall and germinate.

510. The Florideæ or Rose-red series of marine Algæ (which, however, are sometimes green or brownish) are the most attractive to amateurs. The delicate Porphyra or Laver is in some countries eaten as a delicacy, and the cartilaginous Chondrus crispus has been largely used for jelly. Besides their conceptacles, which contain true spores (Fig. 560), they mostly have a fructification in Tetraspores, that is, of spores originating in fours (Fig. 559).

511. The Grass-green Algæ sometimes form broad membranous fronds, such as those of the common Ulva of the sea-shore, but most of them form mere threads, either simple or branched. To this division belong almost all the Fresh-water Algæ, such as those which constitute the silky threads or green slime of running streams or standing pools, and which were all called Confervas before their immense diversity was known. Some are formed of a single row of cells, developed each from the end of another. Others branch, the top of one cell producing more than one new one (Fig. 564). Others, of a kind which is very common in fresh water, simple threads made of a line of cells, have the chlorophyll and protoplasm of each cell arranged in spiral lines or bands. They form spores in a peculiar way, which gives to this family the designation of conjugating Algæ.

512. At a certain time two parallel threads approach each other more closely; contiguous parts of a cell of each thread bulge or grow out, and unite when they meet; the cell-wall partitions between them are absorbed so as to open a free communication; the spiral band of green matter in both cells breaks up; the whole of that of one cell passes over into the other; and of the united contents a large green spore is formed. Soon the old cells decay, and the spore set free is ready to germinate. Fig. 565 represents several stages of the conjugating process, which, however, would never be found all together like this in one pair of threads.