366 Vegetable Physiology. 



tinous secretion from the surface of the stigma. After a short 

 time the outer coat of the pollen-cell appears to burst at one 

 or two points, and to allow the inner coat to pass out through 

 it, in the form of a tube. This tube insinuates itself between 

 the cells of the stigma, and passes down between the long 

 and loosely arranged cells of the style. It gradually extends 

 until it reaches the ovarium itself, even when the style is se- 

 veral inches long. The pollen-grains are not always globular 

 but are sometimes triangular, and emit a pollen-tube at each 

 corner. The tubes, when they arrive at the ovarium, direct 

 themselves towards its different chambers, and have been seen 

 to enter the apertures in the several ovules, which are at that 

 time directed towards the part of the base of the style from 

 which the pollen-tubes project themselves, Sometimes a con- 

 siderable change in the position of the ovule is necessary, in 

 order that the forameu should be applied to the right portion 

 of the wall of the ovary ; but this change always takes place 

 just as the pollen-tubes are passing down the style. The 

 granules which the pollen-grain originally contained, are seen 

 to pass down the tube, and by it some of them are conveyed 

 into each ovule. Whilst yet within the tube, they are seen to 

 develope themselves into new cells, and these cells are the ru- 

 diment of the future plant. 



The germs are thus conveyed into a sort of receptacle, 

 where they are supplied with nourishment that has been pre- 

 viously prepared and stored up for their use by the parent 

 structure ; and they are thus greatly assisted in their early de- 

 velopement. The pulpy matter contained in the ovules, con- 

 sists of starch and sugar ; and these nutritious substances are 

 absorbed by the cells of* the embryo, which increase at their 

 expense. The first' increase of these cells does not so much 

 tend, however, to form those parts which are afterwards to be 

 developed into the stem, root, and leaves, as to produce those 

 temporary structures, termed cotyledons or seed-leaves, which 

 are destined to assist for a time in the developement of the 

 permanent structure, and then to wither and decay. Hence, 

 at the time of the ripening of the seed, the cotyledon forms 

 the greatest part of the embryo or young plant. The starch 



