504 



BOTANY OF THE LIVING PLANT 



presence of external water is essential for fertilisation in Ferns. 

 Their normal life-cycle cannot be completed without it. 



The immediate consequence of fertilisation is growth and segmenta- 

 tion of the zygote, which first secretes a cell-wall. It divides into 

 octants, four of which constitute an epibasal hemisphere, directed 

 towards the apex of the parent thallus, giving rise to axis and leaf of 

 the sporeling ; four form a hypobasal tier, which gives rise to the 

 first root and a suctorial organ called the foot. These parts are soon 

 distinguishable by their form and structure, and are seen in their 



Fig. 397. 

 Embryo of Adiantum continuum in the enlarged venter of the archegonium, so far 

 advanced as to show the parts of the embryo. The epibasal hemisphere is to the 

 left, the hypobasal to the right. L=leaf or cotyledon. K=root. 5 = stem. 

 F =foot. (After Atkinson.) 



relative positions, but still enclosed in the enlarged venter of the 

 archegonium, in Fig. 397. Soon the cotyledon and first root burst 

 their way out : the former expands as the first nutritive leaf, the latter 

 buries itself in the soil (Fig. 399). At first the young Fern-Plant is 

 dependent upon the prothallus that encloses it, but by means of 

 its cotyledon and its root it soon becomes self-dependent, and the 

 prothallus rots away. It is then only a matter of time and opportunity 

 for it to attain characters similar to those of the parent Fern-Plant. 



These are the salient features in the life-cycie of a Fern as it is seen 

 in its simplest form. They may be represented graphically to the 

 eye in a diagram (Fig. 400, p. 506). The two most notable points are 

 those where the individual is represented only by a single cell, viz. the 



