284 EARLY DEVELOPMENT OF THE EMBRYO. 



which the pollen-tubes project themselves. Sometimes a consi 

 derable change in the position of the ovule is necessary, in order 

 that the foramen 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 some of them are conveyed by it, into each ovule. 

 Whilst yet within the tube, they are seen to develope themselves 

 into new cells ; and these cells form the rudiment of the future 

 plant. 



439. 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 

 development. 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 (. 21), which are 

 destined, like the primary frond of the Ferns, to assist for a time 

 in the development of the permanent structure, and then to 

 wither and decay. Hence, at the time of the ripening of the 

 seed, the cotyledon (which is sometimes double, sometimes single 

 see . 440 2) forms the greatest part of the embryo or young 

 plant. Besides this, the seed contains a considerable quantity of 

 starch, destined for the nourishment of the young plant, when it 

 is beginning to sprout, and whilst yet unable to take in food for 

 itself. This starch is sometimes absorbed into the tissue of the 

 cotyledons, rendering them thick and fleshy, as in the Pea or 

 Bean ; and then these, with the small germ to which they belong, 

 form the entire contents of the seed. In other instances, how- 

 ever, the cotyledons are thin leafy organs, and occupy, with the 

 germ, but a small part of the seed ; the remainder then consists 

 of a separate store, which closely resembles the yolk-bag of the 

 eo-or, and is termed the albumen. This is the case in the seeds 



eo' 



