335 



gonium, and the inner mouth of the archegonial canal closes 

 by swelling of the adjacent cells. The fertilized germinal 

 vesicle enlarges until it fills the central cell and then begins 

 to divide, and from it the permanent stem of the fern com- 

 mences its growths, throws out radicles, and the prothallium 

 withers away. 



Passing next to Phcenogamous plants, we find the repro- 

 ductive organs evident enough, polleniferous stamens and 

 ovuliferous pistils ; but we will first take the Gymnosperms 

 or Cycads and Conifers, in which impregnation takes place 

 by direct contact between the pollen and ovule, in Cycads 

 the naked ovules being placed on the margin of modified 

 leaves, while in Conifers the scales covering the seeds are 

 regarded as bracts. 



In Conifers, the pollen grains are applied to the large 

 micropyle of the ovule without the intervention of a stigma, 

 their tubes then traverse the delicate cells of the nucleus and 

 reach the embryo-sac ; the sac enlarges and very slowly 

 becomes filled with endosperm cells. Next, three to six cells 

 at the micropylar end enlarge and form corpuscles, each of 

 which is at first separated from the inner wall of the embryo- 

 sac by a simple cell which divides into four by cross walls, 

 and in the middle of these a passage is left leading to the 

 corpuscle. The pollen-tubes keep slowly growing, pass 

 through the tissue of the nucleus, then perforate the wall of 

 the embryo-sac, and on between the four cells to the cor- 

 puscle, which they impregnate ; a cell at the lower end of 

 the corpuscle then enlarges and forms the embryonal vesicle, 

 a free cell in which di^ddes into eight cells, forming a short 

 cylindric pro-embryo. The four lower cells of this, by elon- 

 gation of the upper, are pushed into the nucleus, the upper 

 now appearing as suspensors, each of the cells at their lower 

 ends becoming an embrj^o. There are thus four times as 

 many embryos as corpuscles, but usually only one developes 



