THE PTERIDOPHYTA : LYCOPSIDA, ETC. 593 



and a tuft of long, quill-like leaves of pale green above. Each leaf bears a 

 ligule, and most of the leaves have a sporangium at the base. The sporangia 

 are heterosporous. In the above points it is clearly related to Se/aginella, 

 though there is no formation of special sporangial strobili, as in the latter 

 genus. As there is no branching and almost all the leaves are sporophylls, 

 the whole plant may be regarded as forming a single strobilus. The anthero- 

 zoids are multiciliate, and this character shows a relationship to the Ferns, 

 so that the systematic position of the group is somew'hat obscure and isolated. 



If we examine a plant towards the end of the year and remove the leaves, 

 starting from the outside, we find, in succession, firstly, sporophylls with 

 megasporangia, then sporophylls with microsporangia and, lastly, 

 vegetative leaves, not yet fully developed, but otherwise exactly like the 

 sporophylls. 



The sporophylls are shed in winter and the plant begins the year with only 

 the immature vegetative leaves. Megasporophylls and microsporophylls 

 are developed in succession during the summer and, while the old leaves drop 

 off, the new vegetative leaves for next season develop nearest the apex. 

 Fertile sporophylls are not formed until the plant is four years old. 



Anatomy of the Stem 



The stem is usually less than 2 cm. high but is proportionally somewhat 

 broader. Stripped of its appendages it is seen to consist of two, or rarely 

 three, broad lobes, with a wide groove between them on each side of the 

 stem. The two grooves meet below to form a cleft across the base. Seen 

 from above the outline is thus markedly flattened, the diameter being 

 roughly twice as great in the line of the lobes as it is across the grooves. The 

 growing point is sunk in an apical depression due to the expansion of the 

 lobar tissues. 



The leaf rudiments arise alternately on opposite sides of the apex, although 

 this simple arrangement is somewhat obscured by the very broad sheathing 

 bases of the older leaves. Some twisting also occurs later, nevertheless the 

 distichous or two-ranked order can be detected even among the mature 



leaves. 



A transverse section of the stem shows a central protostele of peculiar 

 structure (Fig. 602). The centre is occupied by a mixed tissue of short, 

 reticulate tracheids and thin-walled parenchyma, which apparently represents 

 the xvlem. Around this is a laver of prismatic cells, which have none of 

 the histological characters of phloem, but as this layer is continuous with 

 the phloem of the leaf traces it probably represents that tissue in the stem. 

 A thin cambium lies outside this layer, as it does in some Monocotyledons, 

 and it forms secondarv tissues on both sides. Inwards it produces a small 

 number of additional prismatic cells, arranged in rings, which are probably 

 annual. Mingled with these, however, are imperfectly formed tracheids, 

 so that this tissue is unlike anything called phloem elsewhere. On the outside 

 is formed an extensive zone of parenchyma, or secondary cortex (Fig. 603), 

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