DIV. i PTERIDOPHYTA 523 



secondary growth in thickness, bore superposed whorls of, usually six, wedge-shaped 

 or dichotomously-divided leaves. The spike-like cones resembled somewhat those 

 of Equisetum ; each sporophyll bore one to four homosporous sporangia (Fig. 490). 



CLASS IV 

 Lyeopodinae (Club Mosses) ( l > 92 112 > 115 ) 



The Lyeopodinae are sharply distinguished from the other Pteri- 

 dophyta, by their general habit and the mode of their sporangial 

 development. 



They were abundantly represented in the palaeozoic period and 

 included arborescent forms belonging mainly to the extinct orders of 

 Sigillariaceae and Lepidodendraceae. 



The numerous existing species are all herbaceous plants. The 

 most important genera, representing as many orders, are Lycopodium, 

 Selaginella, and Isoetes. 



The dichotomous branching of the stem (Figs. 139, 141) and root and 

 the simple form of the leaves are characteristic of the sporophyte. 

 The two first-named genera have elongated stems and small leaves ; 

 Isoetes, on the other hand, has a tuberous stem and long awl-shaped 

 leaves. Unlike the fertile leaves of the Filicinae and Equisetinae, 

 which always bear numerous sporangia, the sporophylls of the 

 Lyeopodinae produce the sporangia singly, at the base of the leaves 

 or in their axils. Although in many cases scarcely distinguishable 

 from the sterile leaves, the sporophylls are frequently distinctively 

 shaped, and, like those of Equisetum, aggregated at the ends of the 

 fertile shoots into terminal spike -like cones or flowers. Compared 

 with the leaves, the sporangia are relatively large and have a firm 

 wall of a number of layers of cells. The innermost layer of the 

 sporangial wall, the tapetal layer, is not absorbed. On this account no 

 perispore is deposited on the spore-wall. The developing spores are 

 surrounded with a mucilaginous nutritive fluid. The sporangia have 

 no annulus. Except in the case of Isoetes, the spores of which become 

 free by the decay of the sporangial wall, the sporangia dehisce by longi- 

 tudinal slits, which divide the wall into two valves ; the slits occur 

 where rows of cells of the wall have remained thin. Lycopodium is 

 homosporous, while Selaginella and Isoetes are heterosporous. The hetero- 

 sporous forms produce only greatly modified and reduced prothallia ; in 

 the genus Lycopodium, on the other hand, the prothallia are well de- 

 veloped, and show certain resemblances to those of the Ophioglossaceae. 

 The simplified prothalli of Selaginella and Isoetes may be compared to 

 early stages of the prothalli of Lycopodium which have proceeded to 

 form gametes early without undergoing vegetative development. 



The Lycopodiaceae and the Selaginellaceae agree in the segmentation of the 

 embryo, which in both is characterised by possessing a suspensor, and in the 



