THE PTERIDOPHYTA : LYCOPSIDA, ETC. 587 



elongated body, at one end of which two long flagella are produced. The 

 antherozoids escape by the bursting of the thin cell wall at the upper end of 

 the microspore and swim away in the film of water which normally covers 

 the damp soil in which these plants live. The development of the male 

 prothallus often begins before the spores are shed and is completed on the 

 ground. 



The Female Gametophyte 



Meanwhile the megaspores have also germinated. In fact their develop- 

 ment also has already begun before the megaspores have been shed from the 

 megasporangium. 



At an early stage of development the protoplast contracts to a small 

 sac and the wall rapidly expands, so that a wide space separates them. At 

 the same time the spore wall separates into two thick layers, the exospore 

 and the mesospore, separated by a space containing a gelatinous membrane. 

 At this stage there is only one monoploid nucleus, but this soon begins to 

 divide repeatedly until a large number of nuclei are distributed round the 

 protoplasmic layer. The protoplast now rapidly increases both in size and 

 thickness, pressing the two spore coats into contact once more and gradually 

 filling up all the central vacuole with cytoplasm. 



At the apex of the spore, that is towards the centre of the tri-radiate 

 ridge, delicate walls begin to appear in the cytoplasm, dividing it into cells, 

 which form a lens-shaped layer, three cells thick in the middle but only one 

 cell thick at the sides. This is the female prothallus (Fig. 595). Below 



Archegonium 



Prothallus 

 Diaphragm 



Reserve 

 tissue 



Megaspore 

 wall 









'> 



/, 



J 







^ , //• .. ^i^'^>V^i^ 









'* . ^l 



♦*«ri 







Fig. 596. — Selaginella sp. Vertical section of a megaspore 

 containing developed female prothallus. 



