POLLINATION 121 



numbers, can be carried down into the heart of the cone 

 to the bases of the scales on which the ovules are seated, 

 as already described. There is no stigma and the 

 ovules are exposed; hence the pollen grain is dropped 

 upon the micropyle of the ovule, down which it is 

 drawn into a little space just above the body of the 

 ovule itself and inside its integument. This little 

 cavity is known as the pollen chamber. 



In all cases these mechanisms bring the two spores 

 very near to one another. In the Gymnosperms they 

 are separated by only the substance of the upper part of 

 the ovule, whose spore or embryo sac lies quite near the 

 upper end. In the other flowering plants the two 

 spores, the pollen grain and the embryo sac, are separ- 

 ated by the length of the style, the chamber of the 

 ovary, and the upper part of the ovule. 



The next steps in the life history are the germination 

 of these two spores. We have already seen that the 

 result of the germination is the production of little 

 besides reproductive cells, the vegetative parts of the 

 prothalli being very small indeed. In the Gymno- 

 sperm the megaspore or embryo sac becomes filled with 

 a cellular prothallus the endosperm at the upper 

 end of which several archegonia make their appear- 

 ance, each containing one ovum. The pollen grain or 

 microspore puts out a tube which bores its way through 

 the substance of the upper part of the ovule till it 

 reaches the embryo sac and comes into contact with its 

 wall. As it grows it produces two sperms in its interior ; 

 these are for the most part amorphous pieces of proto- 

 plasm, though in a few cases each bears a band of cilia. 

 When the tube reaches the embryo sac the walls at the 

 point of contact dissolve and the sperms pass through 

 and each can fuse with an ovum in one of the archegonia. 



In the angiospermous plants the process is similar, 

 but there are characteristic differences. The tubular 



