POLLINATION 117 



general arrangements of the cones are similar to those 

 of the smaller ones. There is a central axis round which 

 the leaves or sporophylls are spirally arranged. Each 

 sporophyll has on its inner face a flattened outgrowth 

 which in some cases becomes larger than the sporophyll 

 itself. On its upper side this so-called ovali/erous scale 

 bears two sporangia or ovules. There is no closing up 

 of the sporophyll to form an ovary hence the name of 

 the group gymnosperm, meaning naked seed. Each ovule 

 has much the same structure as that described above, but 

 there is only one integument and the micropyle is larger. 



CHAPTER XVI 



POLLINATION AND ITS MECHANISMS FERTILISATION 



THE separation of the spores and the diminution of the 

 phase of the plant which results from their germination, 

 consisting of little more than the ova and the sperms, 

 have made it impossible for fertilisation to take place by 

 means of free-swimming sperms. To ensure its occur- 

 rence it has become necessary to secure that the two 

 spores producing ova and sperms respectively shall be 

 brought close to each other, so that, when they germinate, 

 their prothalli, now so rudimentary, shall be able to 

 meet, in order that the sperm may make its way to the 

 ovum. This has been effected by the transport of the 

 pollen grain the small spore to that part of the flower 

 which bears the large spore or embryo sac. This trans- 

 ference of the pollen is spoken of as pollination. In the 

 fully organised flowers of Dicotyledons and Monocoty- 

 ledons the pollen grain is deposited on the stigma in 

 the cones of the Gymnosperms it reaches the ovule or 

 megasporangium itself. The problem of pollination 

 presents many interesting features which we may now 

 briefly examine. 



