PTERIDOPHYTA 



309 



may be very little modified e.g. L. lucidulum (Fig. 274) or they may form 

 cones at the ends of the shoots. In the latter case, the sporophylls have little 

 chlorophyll, and are broader and shorter than the foliage leaves. 



The young sporangium consists of a group of cells near the base of the young 

 sporophyll. The central cells of the group, by periclinal divisions, give rise to 

 the hypodermal archesporiuin from which later the tetrahedral spores are derived. 

 The limits of the sporogenous tissue are not clearly defined. The wall of the 

 ripe sporangium consists of three layers of cells, of which the inner one is the 

 tapetum. Unlike the tapetum of the other Pteridophytes, the cells here do not 

 become broken down. The sporangium opens by a longitudinal cleft. 



Gemmae 



In Lycopodium selago 

 and L. lucidulum, pecul- 

 iarly modified buds, or 

 gemmae (Fig. 274), are 

 often produced in large 

 numbers. The lower 

 leaves of these buds are 

 thick and fleshy, and 

 protect the young stem- 

 apex until the buds are 

 ready to grow after they 

 are detached. 



Phylloglossum 



Phylloglossum Drum- 

 mondii is a little Aus- 

 tralian plant, evidently 

 related to Lycopodium, 

 and having a striking 

 resemblance to the young 

 sporophyte of L. cer- 

 nuum, so that it has been 

 thought that it may rep- 

 resent the primitive type 

 of the order. Unfortu- 

 nately nothing is known 

 of the gametophyte and 

 embryo. 



Order II. Psilotineae 



This is a small order of 

 mostly tropical plants, 

 represented in our terri- 



B 



FIG. 275. Psilotum triquetrum. A, dichotoinously 

 branched aerial shoot, growing from the rootlike 

 rhizome (X i). B, branch with synangia, sy, 

 slightly enlarged. C, trilocular synangium, with 

 two-lobed sporophyll below it. (After P.I- K- 

 TRAND.) 



