458 



BOTANY 



Seeds, when ripe, consist of the seed-coat (testa and tegumen), 

 embryo, and nutritive tissue. The nutritive tissue is not, however, 

 found in all cases. 



The seed-coat is variously constructed, usually hard and dry ; it 

 is sometimes invested by a fleshy aril developed from the chalaza. 

 The nutritive tissue, or so-called albumen, either takes the form of 

 a perisperm derived from the nucellus (Fig. 363), or, as is more 

 frequent, it is represented by the endosperm. A seed may at the 

 same time be provided with both a perisperm and an endosperm. 

 Both tissues usually consist of a thin-walled parenchyma, the cells of 

 which are packed with reserve material, aleurone grains, starch, fat, 

 etc., to serve for the nourishment of the embryo (Fig. 392). In the 



Fig. 392. — Part of section through one of the coty- 

 ledons of the Pea, showing cells with reserve 



material, am, Starch grains ; al, aleurone grains ; Fid. 393.— Cell from the endosperm of Pliyt- 



p, protoplasm; n, nucleus;.™, cell-wall; i, inter- eUphas macroearpa, with reserve cellu- 



cellular space. lose, (x 340.) 



absence of special nutritive tissues this function is performed by the 

 cotyledons, which then exhibit a similar structure. Sometimes, as in 

 the endosperm of Phytelephas macrocarpa (Fig. 393), valuable technically 

 as vegetable ivory, the cell-walls of the nutritive tissue are enormously 

 thickened; they consist of nearly pure cellulose, and are converted 

 during germination into soluble food materials. 



On germination the cotyledons may remain within the seeds in 

 the ground (hypogean, e.g. in the Pea), or, appearing above the 

 surface of the soil, they may unfold and turn green (epigean, e.g. the 

 Lupine). In the latter case they are frequently more or less leaf-like 

 in character, but they always differ in form and structure from the 

 ordinary foliage-leaves. 



