Fossil plants, Vol. 2

LYCOPODIALES.

The recent members of the Lycopodiales are considered apart from the extinct genera in order that our examination of the latter may be facilitated by a knowledge of the salient characteristics of the surviving types of this important section of the Pteridophyta. A general acquaintance with the extinct as well as with the recent genera will enable us to appreciate the contrasts between the living and the fossil forms and to realise the prominent position occupied by this group in the Palaeozoic period, a position in striking contrast to the part played by the diminutive survivors in the vegetation of the present day. In the account of the recent genera special attention is drawn to such features as afford a clue to the interpretation of the fossils, and the point of view adopted, which at times may appear to lead to an excessive attention to details, is necessarily somewhat different from that represented in botanical text-books[83].

The existing plants included in the Lycopodiales are in nearly all cases perennial herbaceous pteridophytes, exhibiting in their life-histories a well marked alternation of generations. The sporophyte (asexual generation) is characterised by the relatively small size of the leaves except in the genus Isoetes (fig. 132) and in the Australian and New Zealand genus Phylloglossum. The stems are usually erect or trailing, pendulous in epiphytic species or small and tuberous in Isoetes and Phylloglossum. The repeated forking of the shoots (monopodial and dichotomous branching) is a prominent feature of the group. The vascular tissue of the stem usually assumes the form of a single axial strand (stele) (fig. 125), but the shoots of some species of Selaginella often contain two or more distinct steles (fig. 131). The group as a whole is characterised by the centripetal development of the xylem composed almost entirely of scalariform tracheids: secondary xylem and phloem of a peculiar type occur in Isoetes, and the production of secondary xylem elements in a very slight degree has been noticed in one species of Selaginella (S. spinosa)[84]. The roots are constructed on a simple plan, having in most cases only one strand of spiral protoxylem elements (monarch structure). In Lycopodium, in which stem and root anatomy are more nearly of the same type than in the majority of plants, several protoxylem strands may be present. The sporangia are axillary or, more frequently, borne on the upper surface of sporophylls, which are either identical with or more or less distinct from the foliage leaves; in the latter case the sporophylls often occur in the form of a well defined strobilus (cone) at the tips of branches.

The gametophyte (sexual generation) is represented by prothalli which, in the homosporous genera, may live underground as saprophytes, or the upper portion may develop chlorophyll and project above the surface of the ground as an irregularly lobed green structure (e.g. Lycopodium cernuum)[85]. In the heterosporous forms the prothalli are much reduced and do not lead an independent existence outside the spore by the membrane of which they are always more or less enclosed. The sexual organs are represented by antheridia and archegonia; the male cells are provided with two cilia except in Isoetes which has multiciliate antherozoids like those of the ferns.

The existing Lycopods, though widely distributed, never grow in sufficiently dense masses to the exclusion of other plants to form a conspicuous feature in the vegetation of a country. The inconspicuous rôle which they play among the plant-associations of the present era affords a striking contrast to the abundance of the arborescent species in the Palaeozoic forests of the northern hemisphere.

Lycopodiaceae. Lycopodium, represented by nearly 100 species, forms a constituent of most floras: epiphytic species predominate in tropical regions, while others flourish on the mountains and moorlands of Britain and in other extra-tropical countries. For the most part Lycopodium exhibits a preference for a moist climate and appears to be well adapted to habitats where the amount of sunlight is relatively small and the conditions of life unfavourable for dense vegetation. Mountains and islands constantly recur as situations from which species have been recorded. Some species are essentially swamp-plants, e.g. Lycopodium inundatum, a British species, and L. cruentum from the marshes of Sierra Nevada. A variety of the American species, L. alopecuroides (var. aquaticum) affords an instance of a submerged form, which has been collected from an altitude of 12–14,000 ft. on the Andes and Himalayas. It is noteworthy that a considerable variety of habitats is represented within the limits of the genus and that many species are sufficiently hardy to exist in circumstances which would be intolerable to the majority of flowering plants[86].

The British species frequently spoken of as Club Mosses, include Lycopodium Selago, L. annotinum, L. clavatum, L. alpinum, and L. inundatum.

Selaginellaceae. The species of Selaginella, over 300 in number, are widely spread in tropical and subtropical forests, growing on the ground with trailing, suberect or erect stems climbing over taller and stouter plants or as pendulous epiphytes on forest trees.

Selaginella lepidophylla, a tropical American type, popularly known as the Resurrection plant, and often erroneously spoken of as the Rose of Jericho[87], possesses the power of rolling up its shoots during periods of drought and furnishes an example of a species adapted to conditions in marked contrast to those which are most favourable to the majority of species.

The only British species is Selaginella spinosa named by Linnaeus Lycopodium selaginoides and occasionally referred to as Selaginella spinulosa A. Br. (not to be confounded with a Javan species S. spinulosa Spring[88]).

Isoetaceae. Isoetes (fig. 132), of which Mr Baker in his Handbook of the Fern-Allies enumerates 49 species, is a type apart, differing in habit as in certain other characters from the other members of the Lycopodiales. Some botanists[89] prefer to include the genus among the Filicales, but the balance of evidence, including resemblances between Isoetes and extinct Lycopodiaceous plants, would seem to favour its retention as an aberrant genus of the group Lycopodiales. Some species are permanently submerged, others occur in situations intermittently covered with water, and a few grow in damp soil. Isoetes lacustris is found in mountain tarns and lakes of Britain and elsewhere in Central and Northern Europe and North America. Isoetes hystrix[90], a land-form occurs in Guernsey, North-East France, Spain and Asia Minor.

Lycopodiaceae.

The monotypic genus Phylloglossum, represented by P. Drummondii of Australia and New Zealand, though interesting from the point of view of its probable claim to be considered the most primitive type of existing Lycopodiaceous plants, need not be dealt with in detail. A complete individual, which does not exceed 4 or 5 cm. in length, consists of a very small tubercle or protocorm bearing a rosette of slender subulate leaves and prolonged distally as a simple naked axis which overtops the foliage leaves and terminates in a compact cluster of small scale-like sporophylls, each subtending a single sporangium[91].

Lycopodium. It would be out of place in a volume devoted mainly to fossil plants to attempt a comprehensive account of the general morphology of recent species, and indeed our knowledge of the anatomical characters of the genus is still somewhat meagre. For purposes of comparison with extinct types, it is essential that some of the more important morphological features of existing species should be briefly considered. The additions made to our knowledge of the gameophyte[92] of European and tropical species during the last two decades have revealed a striking diversity in habit.

In several species, grouped round the widely distributed type Lycopodium Selago Linn., the comparatively short, erect or suberect, shoots form fairly compact tufts; the ordinary foliage-leaves function as sporophylls, and the sporangia are not localised on special portions of shoots. From this type, we pass to others in which the fertile leaves tend to be confined to the tips of branches, but hardly differ in form from the sterile. A further degree of specialisation is exhibited by species with well-defined cones composed of leaves (or bracts), the primary function of which is to bear sporangia and to afford a protective covering to the strobilus[93].

Lycopodium rufescens Hook. An Andian species with stout dichotomously branched erect stems bears on the younger shoots crowded leaves with their thick and broadly triangular laminae pointing upwards, but on the older and thick shoots the laminae are strongly reflexed (fig. 121, A). The lower part of the specimen represented in fig. 121, A, shows tangentially elongated scars and persistent leaf-bases or cushions left on the stem after the removal of the free portions of the leathery leaves, a surface-feature which also characterises the Palaeozoic genus Lepidodendron. The reflexed leaves and persistent leaf-cushions are clearly seen in the piece of old stem of Lycopodium dichotomum Jacq., a tropical American species reproduced in fig. 121, B. Such species as L. erythraeum Spring, and others with stiff lanceolate leaves exhibit a striking resemblance to the more slender shoots of some recent conifers, more especially Araucaria excelsa, A. Balansae, Cryptomeria, Dacrydium and other genera.

In Lycopodium tetragonum Hook., (fig. 121, C), a species from the Alpine region of the Andes, the long, pendulous and repeatedly forked branches bear four rows of fleshy ovate leaves and simulate the vegetative characters of certain conifers.

L. squarrosum Forst. (fig. 122) a tropical species from India, Polynesia, and other regions, is characterised by its stout stems reaching a diameter of 2·5 cm., bearing long pendulous branches with large terminal cones composed of sporophylls differing but slightly from the foliage leaves. The plant represented in the photograph serves as a good illustration of the practical identity in habit between Palaeozoic and recent genera.

L. Dalhousianum Spring, from the mountains of the Malay Peninsula and Borneo, has larger leaves of finer texture with a distinct midrib reaching a length of 2–3 cm. (fig. 121, E). Another type is illustrated by L. nummularifolium Blume, also a Malayan species, in which the leaves are shorter, broadly oblong or suborbicular, and the branches terminate in narrow and often very long strobili (sometimes reaching a length of 30 cm.) with small bracts in striking contrast to the foliage leaves (fig. 121, D). A similar form of long and slender strobilus occurs in L. Phlegmaria Linn., a common tropical Lycopod: the frequent forking of the strobili noticed in this and other species is a character not unknown among fossil cones (Lepidostrobi).

L. cernuum Linn. (fig. 123), another widely spread tropical type, offers an even closer resemblance than L. squarrosum to the fossil Lepidodendra. The stiff erect stem, reaching in some cases a length of several feet, bears numerous repeatedly forked branches, with crowded linear leaves, terminating in short cylindrical cones with broadly ovate sporophylls. A similar habit characterises the North American species L. obscurum Linn. (fig. 124) bearing cones several centimetres in length.

L. casuarinoides Spring (fig. 121, F) an eastern tropical species, is worthy of notice as exhibiting a peculiar form of leaf consisting of a very small lamina, 3 mm. in length, borne on the top of a long decurrent base, which forms a narrow type of leaf-cushion, bearing some resemblance to the long and rib-like cushions of certain species of Sigillaria, and recalling the habit of slender fossil twigs referred to the Coniferae under such names as Widdringtonites, Cyparissidium, Sphenolepidium.

L. volubile Forst. (fig. 121, G) a New Zealand species, in habit and leaf-form bears a close resemblance to the Jurassic Lycopodites falcatus Lind. and Hutt. (fig. 137): it is also a representative of a few species of Lycopodium which agree with the majority of species of Selaginella in having two kinds of sterile leaves, comparatively long falcate leaves forming two lateral rows and smaller appressed leaves on the upper surface of the branches.

These examples suffice to illustrate the general appearance presented by the vegetative shoots of recent species of which the foliage leaves vary considerably—from the small scale-leaves of Lycopodium tetragonum, to the very slender linear subulate leaves of such a species as L. verticillatum Linn. or the long and broader lamina of L. Dalhousianum (fig. 121, E). It is obvious that fragments of the various types preserved as fossils might well be mistaken either for some of the larger mosses or for twigs of conifers. As Dr Bommer[94] has pointed out in his interesting paper on “Les causes d’erreur dans l’étude des empreintes végétales” some dicotyledonous plants may also simulate the habit of Lycopods: he cites Phyllachne clavigera Hook (Candolleaceae), Tafalla graveolens Wedd (Compositae) and Lavoisiera lycopodioides Gard. (Melastomataceae). Another point illustrated by fig. 121 is the close agreement in habit and in the form of the leaves and leaf-cushions between the recent plants and the Palaeozoic Lepidodendreae.

In his masterly essay “On the vegetation of the Carboniferous Period, as compared with that of the present day” Sir Joseph Hooker called attention to the variation in the shape and arrangement of the leaves in the same species of Lycopodium. The three woodcuts which he publishes of Lycopodium densum, a New Zealand species, afford striking examples of the diversity in habit and leaf-form and justify his warning “that if the species of Lepidodendron were as prone to vary in the foliage as are those of Lycopodium, our available means for distinguishing them are wholly insufficient[95].”

As we have already noticed, there is a considerable diversity among recent species, both as regards habitat and habit; in the anatomy of the stem also corresponding variations occur within the limits of a well-defined generic type of stele. In species with creeping stems, such as L. clavatum[96], the stele exhibits an arrangement of vascular tissue characteristic of the plagiotropic forms. The xylem consists of more or less horizontal plates of scalariform tracheae, each surrounded by small-celled parenchyma, alternating with bands or groups of somewhat ill-defined phloem. The protoxylem and protophloem elements occupy an external position (exarch), pointing to a centripetal development of the metaxylem. This centripetal or root-like character of the primary xylem is an important feature in recent as in fossil Lycopods. The close agreement between the roots and stems of recent species in the disposition of the vascular elements also denotes a simpler type of anatomy than occurs in the majority of vascular plants in which stem and root have more pronounced structural peculiarities. A pericycle, 2–6 cells in breadth, encloses the xylem and phloem bands and this is succeeded by an endodermis, 2–3 cells broad, with vaguely defined limits. In L. clavatum, as in L. alpinum, another British species, the broad cortex is differentiated into three fairly distinct regions; abutting on the endodermis is a zone several layers broad of thick-walled cells constituting an inner cortex modified for protection and support; the central region consists of larger and thinner-walled cells adapted for water-storage and aeration; beyond this is an outer cortical zone of firmer and thicker elements. The prominent leaf-bases or leaf-cushions (fig. 125, A, lc) give to the surface of a transverse section a characteristic appearance which presents the closest agreement with that of the younger shoots of Lepidodendron. From the peripheral protoxylem groups small strands of xylem are given off, which follow a steeply ascending course through the cortex to the single-veined leaves. The leaf-traces, in several species at least, are characterised by a mesarch structure (fig. 125, F, G), the spiral protoxylem elements occupying an approximately central position. The mesophyll of the leaves varies in regard to the extent of differentiation into a palisade and spongy parenchyma; in all cases there is a single vascular bundle occasionally accompanied by a secretory duct.

In erect stems of Lycopodium, as represented by L. cernuum (figs. 123, 125, H, I), L. Dalhousianum, L. squarrosum (fig. 122) and many others, the stele presents a characteristic appearance due to the xylem plates being broken up into detached groups or short uniseriate bands with the interspaces occupied by phloem islands. This type of structure bears a superficial resemblance to that in the single stele of certain species of the fern Lygodium[97], but it is distinguished by the islands of phloem scattered through the stele. In other species the xylem tends to assume the form of a Maltese cross (e.g. L. serratum Thbg.) or it may be disposed as V-shaped and sinuous bands terminating in broad truncate ends composed of protoxylem elements. This form of the xylem and the distribution of the phloem groups are shown in fig. 125, D, E, drawn from a section of a plant of Lycopodium saururus Lam.[98] collected by Mr A. W. Hill at an altitude of 15,000 feet on the Andes of Peru. The position of the protoxylem is shown fig. 125, E, px.

While several species possess a cortex of three distinct zones (fig. 125, H, c, c′, c″), in others the extra-stelar tissue is much more homogeneous, consisting of thin-walled parenchyma or in some cases of thick-walled elements; as a general rule, however, there is a tendency towards a more compact arrangement in the inner and outer portions of the cortex as contrasted with the larger and more loosely connected cells of the middle region. In certain types the middle cortex contains fairly large spaces, as in the swamp-species L. inundatum, which with L. alopecuroides exhibits another feature of some interest first described by Hegelmaier[99]. If a transverse section of the stem of L. inundatum be examined the leaf-traces are seen to be accompanied by a circular canal containing mucilage which extends into the lamina of the leaf. In a specimen of L. cernuum[100] obtained at a height of 2500 ft. by Professor Stanley Gardiner in the Fiji Islands, the leaf-traces (fig. 125, B lt) were found to be accompanied for part of their course by a well-marked secretory space (fig. 125, B, s). There is little doubt that the presence of these mucilage canals is directly connected with a certain type of habitat[101] and attention is called to them in view of a resemblance which they offer to a characteristic strand of tissue, known as the parichnos, which is associated with the leaf-traces of Lepidodendreae and Sigillarieae. In the section shown in fig. 125, H, the xylem of the stele forms more continuous bands than is often the case in L. cernuum which has already been described as having its xylem in small detached groups. The presence of the smaller branch-stele (fig. 125, H, b) affords an example of monopodial branching. The outer cortex of L. saururus (fig. 125, C) exhibits a somewhat unusual feature in the distribution of the thicker-walled tissue (b) which encloses a patch of more delicate parenchyma (a) with large lacunae (lc) in the region of the leaf-bases, and presents the appearance of an irregular reticulum. This arrangement of the mechanical tissue in the outer cortex is comparable with that in stems of some species of Sigillaria.

In certain species of Lycopodium the roots[102], which arise endogenously from the axial vascular cylinder, instead of passing through the cortex of the stem by the shortest route, bend downwards and bore their way in a more or less vertical direction before emerging at or near the base of the aerial shoot. The transverse section of L. dichotomum represented in fig. 125, A, shows several roots (R) in the cortex; they consist of a xylem strand of circular or crescentric form accompanied by phloem and enclosed by several layers of root-cortex. The roots of Lycopodium do not always present so simple a structure as those of L. dichotomum; the xylem may have an irregularly stellate form with as many as ten protoxylem groups.

Reproductive Shoots[103]. In Lycopodium Selago the foliage leaves serve also as sporophylls and, as Professor Bower[104] has pointed out, the branches exhibit to some extent a zonal alternation of sterile and fertile leaves; in other species, in which foliage leaves and sporophylls are practically identical, the sporangia occur sporadically on the ordinary leaves. In species with well-defined terminal cones the lower sporophylls may bear arrested sporangia and thus form transitional stages between sterile and fertile leaves, a feature which occurs also in the male and female flowers of many recent Araucarieae[105]. The sporangia[106] (fig. 126, D, F) are usually reniform and compressed in a direction parallel to the surface of the cone-scales; they are developed from the upper surface and close to the base of the fertile leaf to which they are attached by a short and thick stalk (e.g. L. inundatum) or by a longer and more slender pedicel (L. Phlegmaria, fig. 126, E). On maturity the sporangia open as two valves in the plane of compression and the line of dehiscence is determined in some species at least by the occurrence of smaller cells in the wall. In transverse sections of cones in which the sporangia are strongly saddle-shaped, the sporophylls may appear to bear two sporangia. This is well shown in the section of a cone of L. clavatum shown in fig. 126, F. The sporangia a and b are cut through in an approximately median plane showing the irregular outline of the sterile pad (p) of tissue in the sporogenous cavity. Those at c and d have been traversed at a lower level and the two lobes of the saddle-shaped sporangia are cut below the attachment to the sporophyll. The distal laminae of the sporophylls, cut at different levels, are seen at the periphery of the cone.

In longitudinal radial section of some cones the sporangia appear to occupy an axillary position, but in others (e.g. L. clavatum) they are attached to the horizontal portion of the sporophyll almost midway between the axis of the cone and the upturned distal end of the sporophyll (fig. 126, D). The wall of a sporangium frequently consists of 2–3 cell-layers and in some cases (e.g. L. dichotomum), it may reach a thickness of seven layers, resembling in this respect the more bulky sporangia of a certain type of Lepidodendroid cone. The sporogenous tissue is separated from the stalk of the sporangium by a mass of parenchymatous tissue which may project as a prominent pad (fig. 126, D, F, p) into the interior of the sporogenous cavity. This basal tissue (the subarchesporial pad of Bower[107]) has been observed in L. clavatum to send up irregular processes of sterile cells among the developing spores, suggesting a comparison with the trabeculae which form a characteristic feature of the sporangia of Isoetes and with similar sterile strands noticed by Bower[108] in Lepidostrobus (cone of Lepidodendron).

Each sporophyll is supplied by a single vascular bundle which according to published statements never sends a branch to the sporangium base. The fertile tips of the foliage shoots of L. cernuum (figs. 126, A–C) afford good examples of specialised cones. The surface of the cone is covered by the broadly triangular laminae of sporophylls (fig. 126, C) which in their fimbriate margins resemble the Palaeozoic cone-scales described by Dr Kidston[109] as Lepidostrobus fimbriatus. The distal portions of the sporophylls are prolonged downwards (fig. 126, A) to afford protection to the lower sporangia, their efficiency being increased by the lignified and thicker walls (A, a) of the cells in the lower portion of the laminar expansion. The cells of the sporangial wall are provided with strengthening bands which in surface-view (fig. 126, B) present the appearance of prominent pegs. Since the appearance of Miss Sykes’s paper on the sporangium-bearing organs of the Lycopodiaceae, Dr Lang[110] has published a more complete account of the structure of the strobilus of Lycopodium cernuum in which he records certain features of special interest. The importance of these morphological characters is increased by their agreement, as shown by Lang, with those of the Palaeozoic cone Spencerites[111]. The sporophylls of a cone (12 mm. long by 3 mm. in diameter) of Lycopodium cernuum show an abrupt transition from the foliage leaves, but like these they occur in alternate whorls of five. A large sporangium is attached to the upper face of each sporophyll close to the base of the obliquely vertical distal lamina (fig. 127); each sporophyll, which is supplied with a single vascular bundle, has a large mucilage-cavity (m) in its lower region. “The mucilaginous change” in the sub-sporangial portion of a sporophyll “extends to the surface involving the epidermis, so that this portion of the sporophyll-base may be described as consisting of a mass of mucilage bounded below by a structureless membrane[112].” Dehiscence of the sporangia occurs at the middle of the distal face (fig. 127, x). As seen in the radial section (fig. 127, ma) the outer margin of the base of the sporophyll bears a short outgrowth. The leaf-bases of each whorl hang down between the sporangia of the alternating whorl below, and the base of each sporophyll is coherent with the margins of the two sporophylls of the next lower whorl between which it lies, the sporangia being thus closely packed and lying in a pocket “open only on the outer surface of the cone.” Fig. 128 represents a transverse section through a cone in the plane AA of fig. 127; this traverses the sporangia and their subtending bracts (b) of one whorl and the dependent bases of the sporophylls of the next higher whorl in the region of the mucilage-sacs (m), which are bounded at the periphery by the outer tissue of the sporophylls (a). A transverse section in the plane BB of fig. 127 is shown in fig. 129: the pedicels and a part of each vascular strand are seen at b radiating from the axis of the cone; one sporophyll (sp, a) is cut through in the region of the pad of tracheal tissue that characterises the short sporangial stalks. The upper portions of the sporangia of the next lower whorl, which project upwards against the mucilaginous bases of the sporophylls above (cf. fig. 127, BB) are shown at c and external to them, at a, the section has cut through the outer persistent portions of these sporophyll bases.

As Lang points out, this highly complex structure is an expression of the complete protection afforded to the sporangia of a plant met with in exposed situations in the tropics; it is also of importance from a morphological standpoint as exhibiting an agreement with the extinct type of Lycopod cone represented by Spencerites.

Selaginellaceae.

Selaginella differs from Lycopodium in the production of two kinds of spores, megaspores and microspores, and, in the great majority of species, in the dimorphic character of the foliage leaves, which are usually arranged in four rows, the laminae of the upper rows being very much smaller than those of the lower (fig. 130, 1–3). The smaller leaves are shown more clearly in fig. 130, 1a. It is obvious from an examination of a Selaginella shoot, such as is shown in fig. 130, that in fossil specimens it would often be almost impossible to recognise the existence of two kinds of leaves. Some species, e.g. Selaginella spinosa[113], the sole British representative of the genus, are homophyllous and agree in this respect with most species of Lycopodium. Another feature characteristic of Selaginella, as contrasted with Lycopodium, is the presence of a ligule in both foliage leaves and sporophylls. This is a colourless thin lamina attached by a comparatively stout foot to the base of a pit on the upper surface and close to the lower edge of the leaf (fig. 130, 4, l; fig. 131, E, F, l).

In an erect species, such as S. grandis Moore[114] (fig. 130 and fig. 131, G) from Borneo, the main shoots, which may attain a height of 2–3 feet, bear small and inconspicuous leaves of one kind, but the lateral and repeatedly forked shoots are heterophyllous. The passage from the homophyllous to the heterophyllous arrangement is shown in the transition from the erect to the dorsiventral habit of the lateral shoots (fig. 130, 2). The monopodially or dichotomously branched shoots produce long naked axes at the forks; these grow downwards to the ground where they develop numerous dichotomously forked branches. For certain reasons these naked aerial axes were named rhizophores and have always been styled shoots, the term root being restricted to repeatedly forked branches which the rhizophores produce in the soil. It has, however, been shown by Professor Harvey-Gibson[115] that there is no sufficient reason for drawing any morphological distinction between rhizophores and roots, the term root being applicable to both.

Our knowledge of the anatomy of Selaginella, thanks chiefly to the researches of Harvey-Gibson[116], is much more complete than in the case of Lycopodium. The stems, which may be either trailing or erect, are usually dorsiventral, and it is noteworthy that different shoots of the same plant or even the same axis in different regions may exhibit considerable variation in the structure and arrangement of the vascular tissue. In the well-known species, Selaginella Martensii, the stem, which is partly trailing, partly ascending, possesses a single ribbon-shaped stele composed of scalariform tracheids with two marginal protoxylems formed by the fusion of the leaf-traces of the dorsal and ventral leaves respectively. As in Lycopodium the metaxylem tracheae are as a rule scalariform, but reticulate xylem elements are by no means unknown. The tracheal band, surrounded by parenchymatous elements, is enclosed by phloem with external protophloem elements. The characteristic features of the stele are shown in the diagrammatic drawing of a section of another species—S. Willdenowii—represented in fig. 131, A.