176 Practical Plant Biology. 



segments aggregate in the protoplasm immediately within the 

 three converging flattened sides of the spore. Cell-walls are formed 

 dividing this protoplasmic mass into cells each containing a 

 nucleus, and thus a tissue is formed at the apex of the spore. Later 

 on, by further nuclear divisions followed by the formation of cell- 

 walls, the whole body of the spore is filled with tissue. The growth 

 of this gametophyte bursts open the apex of the spore and exposes 

 the first formed tissue within the spore. In this tissue the arche- 

 gonia are formed. The first to arise are centrally placed on 

 this exposed tissue. If they fail to be fertilised others further from 

 the centre are formed. The archegonia are almost completely 

 sunk in the tissue. They consist of a neck composed of two tiers 

 of four cells each, a canal-cell, a ventral canal-cell and an ovum. 

 The disintegration of the canal-cells opens the way to the ovum 

 and the sperms usually utilising the moisture of the soil on which 

 the microspores and megaspores have been shed enter the neck and 

 fertilise the ovum. 



The oosperm divides transversely (i.e. at right angles to the 

 axis of the archegonium) and forms a short column of cells. 

 Those next the neck of the archegonium form an organ called the 

 suspensor, while the lowest of the series and the furthest from the 

 neck subdivides and becomes the embryo. 



The embryo is at first an ovoid mass of cells at the lower end 

 of the suspensor. The unequal development of one side produces 

 a prominence which becomes the growing end of the young stem. 

 On the sides of this two smaller excrescences are formed ; these 

 are the beginnings of the two first leaves. A third enlargement 

 is formed just opposite the point of attachment of the suspensor. 

 This becomes the foot which acts as an haustorial organ and ab- 

 sorbs the food material stored in the gametophyte. Later on the 

 first root arises in the tissue between the foot and the suspensor. 

 Very soon the growth of the embryo causes it to burst out from 

 the gametophyte and the containing spore coat. Its root grows 

 down into the ground to absorb water and the necessary mineral 

 supplies, while the stem pushes forth producing leaves to carry on 

 photosynthesis and transpiration, and thus the young sporophyte 

 is launched. 



Comparison of Selaginella with Aspidium shows that the 

 gametophyte of the former is much simpler in its construction than 

 that of Aspidium. The vegetative apparatus of the male gameto- 

 phyte is limited to the single small cell formed by the first division 

 of the microspore, and this corresponds to the whole prothallus of 

 Aspidium. The small capsule of cells surrounding the sperm- 



