76 PLANT-LIFE ON LAND [OH. 



on passively by the pollen-tube to the ovum. The 

 detail of method differs in the two cases, but the 

 essentials are the same in both. 



Having thus recognised their fundamental likeness, 

 it lies open to us to enquire whether the differences 

 afford any ground for opinion as to descent? The 

 main difference between the two processes lies in the 

 medium of transfer. In the Fern the spermatozoid 

 moves in water, and external fluid water is necessary 

 for fertilisation. In the Flowering plant it is not. 

 The formation of the pollen-tube and its conveyance 

 of the male gamete to 'the egg may be carried out 

 without reference to external fluid water. It commonly 

 occurs in dry summer weather, and is only dependent 

 on water so far as the whole plant is so dependent for 

 carrying out its ordinary functions. In this respect 

 then the Flowering plant may be said to be essentially 

 a plant of the Land. The Fern, though capable of 

 vegetating on the Land, is dependent at a most 

 critical moment on external water. In this it reveals 

 an amphibious character. 



But a further question which has its immediate 

 bearing on descent is whether there is evidence of 

 transition from one of these types to the other. As 

 the Ferns show many relatively primitive characters 

 both in structure and development this question will 

 present itself only in one way. Is there any evidence 

 of a transition from the aquatic fertilisation by 



