THE ANGIOSPERMAE 



1573 



of germination, which will be reserved for the section on the physiology of 

 growth in Volume III. 



As the absorption of water proceeds, the solution of the reserve foods 

 in the endosperm or embryo begins (Fig. 1433). Many endosperms are very 

 hard in the dry state, and contribute to the mechanical protection of the 



^t^ 



Fig. 1433. — Grains of starch from a germinating Wheat 

 grain, showing the process of digestion. 



embryo, but even the hardest soften considerably as the starch or protein 

 reserves become fully imbibed. The starch-free endosperms, in which the 

 cell walls are highly thickened with "reserve cellulose", swell very strongly 

 and the cells lose their outlines and the tissue becomes slimy and semi- 

 liquid, as in Trigonella, Lotus and many other Papilionaceae. 



The embryo, before dormancy, may absorb and destroy the inner 

 endosperm which is in immediate contact with it, only the outer endosperm 

 being left intact, but the swelling of both endosperm and embryo tissues 

 after imbibition brings the two structures into intimate and pressing 

 contact, which facilitates the transfer of materials from one to the other. 

 The seeds of the Grasses and some other plants, e.g., Fagopyrum, have a 

 cambium-like layer on the periphery of the endosperm, which remains 

 active until the maturity of the seed, cutting off, internally, layers of starchy 

 cells. This activity ceases at maturity, but the cambium cells then divide 

 anticlinally into short, square cells, which become filled with solid protein. 

 This is the so-called aleiirone layer, w^hich is prominent in many of the 



