59° 



A TEXTBOOK OF THEORETICAL BOTANY 



transverse wall. The upper cell, which lies nearest to the neck of the arche- 

 gonium, forms the suspensor ; the lower cell alone forms the embryo. 

 The suspensor cell elongates remarkably, penetrating the diaphragm and 

 carrying the embryo down into the lower part of the spore (Fig. 598). As 

 the embryo develops, the division of the remainder of the spore-protoplasm 



into cells is completed, in a way which clearly recalls 

 the formation of endosperm in the Angiosperms. 



At a slightly later stage we can recognize the 

 following structures in the embryo. At the lower 

 end, opposite the suspensor, lies the stem apex, 

 marked by a wedge-shaped cell, enclosed on either 

 side by the first two leaves or cotyledons. On 

 one side a little further back is a large swollen mass 

 of tissue, the foot, which is the chief organ of food 

 absorption, whilst a little later the radicle becomes 

 differentiated between the foot and the suspensor. 

 The development of the foot pushes the stem apex 

 to one side so that the axis of the embryo is hori- 

 zontal with reference to the suspensor. In this 

 position it develops until both ends of the axis 

 have grown beyond the limits of the prothallus, 

 when the root turns down and the shoot upwards. 

 The embryo remains for some time attached to 

 the megaspore, drawing nourishment from it. It 

 looks very like an Angiospermic seedling attached 

 to a seed (Fig. 599). 



The two cotyledons are rather larger than the 

 subsequent leaves. The stem below them grows 

 until it may be a couple of centimetres long. It 

 is called the hypocotyl. Above the cotyledons 

 the young stem may grow for a short distance 

 unbranched, but it usually divides immediately 

 into two by the division of the apical cell. This 

 is the only true dichotomy in its life history 

 and it is marked by the transformation of the 

 apical cell from a wedge shape into the four- 

 sided cell characteristic of the mature stem. 



Fig. 599. — Selagiuella krous- 

 siana. Young sporeling 

 showing megaspore from 

 which arises a young shoot 

 showing the first dicho- 

 tomy and three rhizo- 

 phores, the longest of 

 which has produced two 

 roots. 



Alternation of Generations 



The most interesting point of diiTerence between Selaginella and the 

 types we have so far considered lies in the presence of two types of spores 

 and the two distinct forms of prothalli which develop in them. The life- 

 cycle, therefore, is more complex than in our previous example and may be 

 represented by the following scheme (Fig. 600) : — 



