192 PHYSIOLOGY OF NUTRITION. 



consists of transparent cells. The seed-coat encloses the endo- 

 sperm and the embryo. To these we return later. It may here 

 be observed merely that the layer of the endosperm immediately 

 within the seed-coat (Kleberzellschiclit) consists of a single layer 

 of almost quadratic cells which, though this, it is true, cannot 

 be made out in the sections treated with potash solution contain 

 aleurone grains (no starch). Then come the more or less starchy 

 tissues of the endosperm. 



We prepare transverse sections of the seed of Lupinus luteus. 

 It is best to use partially soaked seeds and investigate the sections 

 in water and potash solution. The cuticle is fairly well developed, 

 and covered with a granular layer (wax). The epidermis demands 

 special attention. It consists of long palisade cells, arranged 

 radially with respect to the surface of the seed. The walls of 

 these cells are very thick. Groups of the cells contain a brown 

 pigment, which accounts for the spotted appearance of the seeds. 

 Below the epidermis we observe a single layer of columnar cells, 

 with very wide intercellular spaces between them, and arranged 

 at right angles to the surface of the seed. Then follow several 

 layers of tangentially elongated cells, which swell up considerably 

 when treated with potash. Adjoining these are the much com- 

 pressed remains of the endosperm, and finally we come to the 

 tissue of the cotyledons. 



If a number of Lupinus seeds, say 100, are thrown into water, 

 and left there for a good time, e.g. twenty-four hours, or even eight 

 to fourteen days, we shall find that by no means all of them have 

 absorbed water. This resistance to the influence of water, which 

 is of great biological significance, is characteristic of many other 

 kinds of seeds besides those of Lupinus, and in the case before us 

 is due to the fact that the cells of the perfectly intact palisade 

 layer of the seed-coat, owing to special peculiarities of their 

 membranes, are only with great difficulty penetrated by water. 

 If we injure the palisade layer of a lupin seed, it always soaks 

 readily when placed in contact with water. 1 



The seeds of Pisum sativum are among those that soak readily, 

 as we can satisfy ourselves by putting them in water. The testa 

 of the Pisum seed in many ways resembles in its structure that of 

 Lupinus. We peel the seed-coats from soaked seeds, fold them 

 up, and prepare delicate transverse sections, which we examine in 

 caustic potash. Here also the palisade layer is followed by a 

 columnar layer. Then comes a many-layered parenchyma, the 



