42 PLANT-LIFE ON LAND [CH. 



circumstances burst at the projecting tip (Fig. 9, B), 

 and an open channel then leads down to the egg, 

 which is deeply seated in the tissue (Fig. 9, C). 

 It might at first sight appear a matter of remote 

 chance that any individual spermatozoid should 

 enter an archegonium, but this is what may be 

 seen to occur with a high degree of certainty. The 

 explanation lies in the fact that the archegonium 

 gives out by diffiision into the water a very small 

 quantity of a soluble substance which acts as an 

 attraction to the spermatozoids. They move actively 

 from the weaker to the stronger solution, that is to 

 the neck of the archegonium, which is the centre of 

 the diffusion. One or more of them may enter the 

 neck, and passing down it one may be seen to fuse 

 with the egg-cell. This is the act of fertilisation, 

 which consists in the fusion of two cells, and 

 especially of their two nuclei ; for notwithstanding 

 its strange form the spermatozoid is actually a cell, 

 bearing a nucleus which forms the greater part of 

 its spiral body. The result of the fusion is a cell 

 called the zygote. It soon surrounds itself with a 

 cell-wall, and forms the starting point for the 

 development of a new Fern-plant similar to the 

 original parent. The embryo plant is nursed for a 

 time by the prothallus, upon which it lives as a 

 parasite while young. But soon it becomes inde- 

 pendent, rooting itself in the soil, and expanding 



