CRYPTOGAMS 



419 



Selaginella helvetica (Fig. 351), whose stem is covered with two rows of 

 small dorsal or upper leaves, and opposite to them two rows of larger, 

 ventral or under leaves. The development 

 of a small, membranous ligule at the 

 base of the leaves, on their dorsal side, is 

 characteristic of the Selaginellas. 



As in Lycopodium, the cones or flower- 

 spikes are terminal. Each sporophyll 

 subtends only one sporangium, which 

 springs from the stem above the leaf- 

 axil. The same spike bears both kinds of 

 sporangia ; the macrosporangia occur in 

 smaller numbers in the axils of the lower 

 sporophylls. The two kinds of sporangia 

 do not differ so much in size as in form. 

 Each microsporangium (Pig. 352, b) has 

 the form of a flattened capsule, and open- 

 ing in two valves discharges numerous 

 microspores. The macrosporangia (Fig. 

 352, a), on the contrary, are spherical, 

 and each contains only four macrospores, 

 which are produced by the growth and 

 division of a single spore-mother-cell ; all 

 the other mother-cells originally developed fk>. 351.— a, Seiagineiia helvetica (from 

 ultimately disappear. On account of the " at " re - " at s ' z <* B - f <f "?? 



, . - , devticulata, embryonic plant with 



increasing size of the spores the macro- macrospore s tni attached. (After 

 sporangia become inflated and nodular. At Bisohoff, magnified.) 

 maturity they split into several valves. 



The microspores begin their development while still enclosed 



within the sporangium. The spore 

 first divides into a small lenticular 

 vegetative cell representing the pro- 

 thallium, and into a large cell which 

 represents the rudiment of an anther- 

 idium ; the latter divides successively 

 into eight sterile peripheral cells and 

 two or four central spermatogenous 

 cells (Fig. 353, A). By the further 

 division of the central cells numerous 

 spermatozoid mother-cells are formed 

 (B-D). The peripheral cells then 



Fig. 352.- Selaginella helvetica, a, Open foga^ down and give rise to a muci- 

 macrosporangium showing three macro- . . . ° . , . , . 



spores (the fourth hidden from view) ; 6, lagmous substance, m which is em- 



microsporangiuin with escaping micro- bedded the central mass of sperma- 



spores. (x circa 10.) tozoid mot h er -cells (B). The small 



prothallium-cell, however, persists. Eventually the wall ruptures, and 



