THE NATURE OF SEEDS 



347 



cellulose of most plant tissues is not ordinarily used as 

 food, but in some seeds the cell walls of the endosperm 

 as well as of the testa 

 become very thick. 

 This thickening is com- 

 posed of what is called 

 reserve cellulose. (See 

 Figure 155.) Upon 

 germination this re- 

 serve cellulose is used 

 as food for the seed- 

 ling. The seeds of 

 persimmons and dates 

 have much food stored 

 in this way. Reserve 

 cellulose gives excep- 

 tional hardness to the 

 endosperm ; seeds 

 which have much of it 

 are said to be homy. 

 In nearly all seeds 



Fig. 154. — Cross section through the outer part 

 of a grain of wheat, much magnified ; k, the 

 husk whose outer part is pericarp and whose 

 inner part is testa; a, the gluten or aleurone 

 layer whose cells are filled with grains of pro- 

 tein ; b, a partof the starchy tissue which makes 

 up the body of the grain. — After Cobb. 



Fig. 155.— Section through 

 part of the endosperm 

 of a persimmon seed. 

 Note the greatly thick- 

 ened walls of reserve 

 cellulose. 



both nitrogenous and non-nitrogenous 

 foods occur, but the latter are always 

 present in greater amount than the 

 former. Usually, in any one kind of 

 seed, one kind of non-nitrogenous food 

 is present in large excess over all other 

 kinds of food. On this basis seeds 

 may be classed as starchy, oily, or horny 

 as the case may be. The seeds most 

 extensively used for food, including the 

 grains, are starchy seeds. 



