THE PTERIDOPHYTA : LYCOPSIDA, ETC. 6oi 



Before the division of each mother cell the included starch grains arrange 

 themselves into four groups, and the four nuclei resulting from the meiotic 

 divisions place themselves in contact with the four starch groups. This is 

 immediately followed by the formation of new dividing walls. 



The first wall to form round the megaspore is a membrane which later 

 becomes the perispore. Within this is secreted the exospore, which later 

 divides into three layers, the inner two being the mesospore and the 

 endospore respectively. This recalls the megaspore of Selaginella, and 

 in the present plant there is the same separation of the protoplast from 

 the wall during development, which occurs in the former type. The 

 number of megaspores in each sporangium varies but is of the order 



of ICO to 200. 



Among the cells of the archesporium in its early stages there are four 

 distinct conditions. Firstly there are the true spore mother cells. Secondly 

 there are cells which enlarge, like the former, but remain sterile and sub- 

 sequently divide. Thirdly there are the cells which form the tapetum, 

 arranged in three or four layers round the mother cells. Lastly there are 

 rows of cells which remain small and comparatively empty. These last 

 persist in the ripe sporangium as membranes running across the sporangium 

 among the mature spores, and called the trabeculae. All these kinds of 

 cells are apparentlv potentially sporogenous and have been altered by pro- 

 gressive sterilization. 



The trabeculae apparently represent a development of the pro- 

 cesses which arise from the sub-archesporial tissue in the sporangium 

 of Lycopodium, and they have several parallels in the sporangia of 

 species of Lepidodendron. Their function is apparently nutritive like that 

 of the tapetum. 



The mature spores are covered by a dense white perispore, very warty 

 on the surface and marked with a prominent triradiate ridge which determines 

 the lines of dehiscence. 



The sporangial wall is usually two to three cells thick and does not dehisce 

 but liberates the spores by decay. 



Development of the Microsporangium 



The microsporangium is at first identical in structure with the mega- 

 sporangium. The massive archesporium and the sporangial wall are not 

 clearly segregated until a late stage of development. Before the mother cell 

 stage is reached there is an evident differentiation of trabeculae, the cells 

 of which lose most of their protoplasm (Fig. 612). These trabeculae partially 

 divide the archesporial mass into a number of portions, each of which 

 is surrounded by a tapetum, two or three cells thick. The mass of 

 sporogenous cells is ver\^ large and the number of microspores is 

 probably the largest formed in the sporangia of any vascular plant. 

 It is estimated to be as high as half a million in the present species, while 

 20 A 



