THE PTERIDOPHYTA : LYCOPSIDA, ETC. 571 



root, may be due to the much greater development of the leaves and leaf 

 traces in these forms. 



Development of the Strobilus 



The strobili occupy the ends of erect branches called podia, which 

 are radially symmetrical and which bear small leaves spaced at intervals 

 from one another, contrasting sharply with the closely spiral strobilar leaves 

 or sporophylls, upon which the sporangia are produced. Each podium 

 normally bears two strobili, on axes produced by a dichotomy of the apex. 

 The strobili are 3 to 6 cm. long, and being carried at the end of the upright 

 podia are favourably placed for wind dispersal of the spores. 



Each sporophyll bears one large sporangium on its upper surface, close 

 to the base (Fig. 580). All the sporangia are alike and each contains numerous 

 small spores. The sporophylls have broad bases and closely overlap, so that 

 the sporangia are well protected. The sporophylls are adapted only to bear 

 and protect the sporangia. They have little or no chlorophyll, though stomata 

 are present. Each bears a dorsal outgrowth, directed downwards, which 

 fits between the sporangia below and helps to protect them. A remarkable 

 feature of the anatomy of the sporophyll is the presence of an area of dis- 

 integrated cells w^hich appears to represent a parichnos like that in the leaves 

 of Isoetes and Lepidodendron. 



The sporangium arises from a transverse row of cells on the surface of the 

 sporophyll (Fig. 581). These, by division, give rise to a wall, three cells 

 thick, the innermost of which becomes the tapetum or nourishing layer for 

 the spores. Centrally there is the archesporium, consisting of three 

 tangential rows of twelve cells each, from which, by further divisions, arise 

 the spore mother cells. These undergo meiosis and produce tetrads of 

 monoploid spores. Below the archesporium the wall forms a pad of sterile 

 tissue which grows up in the form of cellular processes among the spores 

 and provides the basal portion of the tapetum. The bottom of the sporangium 

 forms a short, thick stalk or sporangiophore. 



The mature sporangium is about 2 mm. across and is kidney-shaped. It 

 opens by a tangential slit across the top and the spores are scattered by the 

 wind. They settle on the ground and each germinates into a prothallus. 

 The spores are yellow and are beautifully reticulated on the outer surface. 

 Each is also marked with a triradiate ridge, along which the wall opens when 

 the spore germinates, which may, however, be delayed for several years. 



Development of the Prothallus 



The prothallus in Lycopoditim clavatiim differs considerably from that in 

 the Ferns. In the first place it is a subterranean structure, solid and tuberous, 

 which takes from six to fifteen years to mature. Like the prothallus of 

 Botrychium it is colourless and saprophytic and contains a mycorrhizal Fungus 

 (Figs. 582 and 583). 



