THE METABOLISM OF PLANTS. 179 



The question has naturally arisen in reference to albumi- 

 nous seeds, as to how far the changes in the reserve-materials 

 of the endosperm are to be ascribed to the embryo. In order 

 to answer this question van Tieghem experimented with the 

 embryo of Mirabilis, depriving it of its endosperm and supply- 

 ing it artificially with nutriment ; under these conditions the 

 embryo grew and developed, but not so well as under normal 

 conditions. From this we may infer that, in the seed, changes 

 go on in the endosperm, independently of the embryo, by 

 which the reserve-materials stored up in it are prepared for 

 absorption by the embryo. The results of the similar ex- 

 periments of Blociszewski on Rye-seeds lead to the same 

 conclusion, and it is further supported by van Tieghem's 

 observation that the isolated endosperm of Ricinus can grow, 

 under favourable conditions, and that the aleurone-grains 

 become disorganised and the oil replaced by starch. There 

 can be no doubt, however, that the embryo exerts an impor- 

 tant influence in bringing about the changes in the reserve- 

 materials of the endosperm, though it is difficult to determine 

 what the exact nature of this influence is. It may be that 

 the embryo excretes a ferment, or it may simply act by 

 removing the products of ferment-action, thus preventing 

 their accumulation and so assisting the processes. 



The difference in structure between albuminous and exal- 

 buminous seeds is essentially this, that whereas in the former 

 the reserve-materials of the endosperm are only absorbed by 

 the embryo during germination, in the latter they are ab- 

 sorbed by the embryo during the ripening of the seed, and 

 are deposited in the cotyledons. 



We will now discuss the structure and composition of the 

 starch-grains and of the aleurone-grains. The starch-grains 

 present evident structure : though they vary in size and shape 

 in different seeds, yet in all cases they present a stratified 

 appearance, consisting apparently of layers deposited concen- 

 trically or excentrically around a certain point, the hilum 

 (Fig. 26) : in fact, as Strasburger points out, the optical section 

 of a grain resembles the transverse section of a thickened 

 stratified cell-wall. They are formed by the protoplasm ; and 



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