146 BRITISH PLANTS 



cereal seeds are starchy ; rhizomes, bulbs, and tubers 

 possess large stores of starch. In a few cases the reserve 

 is sugar : the fleshy root of the beet is loaded with it ; 

 the onion is sugary ; so are some seeds. In other seeds 

 oil is a form of reserve e.g., nuts and in a few cases the 

 reserve is even stored up as cellulose in the cell-walls, 

 which then become enormously thickened e.g., the date- 

 stone. Nitrogenous food-reserves are found chiefly in 

 seeds, but they are seldom so important as carbohydrates, 

 and rarely give rise to conspicuous swollen structures. 



The Seats of Storage of Food-Reserves. 



In what follows, food-reserves in plants are only dealt 

 with in so far as these stores are serviceable to man, either 

 as a source of food, or on account of their value for 



economic purposes. 



1. The Seed. The food- 

 material stored up in seeds 

 is for the use of the young 

 plant during its germina- 

 tion, and during that 

 period of infancy known 

 as the seedling-stage, when 

 FIG. 52. LONGITUDINAL SECTION ft j s una ble adequatelv to 



SEED OF 



a, seed-coat ; 6, endosperm ; c, embryo. albuminous seeds (Figs . 52, 



53, 54) this reserve of food 



lies outside the embryo in the cells of the endosperm 

 e.g., wheat, iris, spindle-tree, poppies, buttercups, and all 

 the Umbelliferse. In exalbuminous seeds (Fig. 55) the 

 food is stored in the embryo itself, usually in the coty- 

 ledons, which are then large e.g., pea, bean, acorn, sunr 

 flower but sometimes in the hypocotyl e.g., brazil-nut, 

 which is, botanically, not a nut at all, but a seed with 

 a hard shelly coat. 



(a) Carbohydrate reserves occur in the form of 



(i.) Starch, in starchy seeds e.g., the cereal grains, 

 wheat, barley, rice, etc., which yield flour when ground. 



(ii.) Sugar in sugary seeds e.g., certain varieties of 

 maize. 



(b) Protein occurs in the form of amorphous particles 

 (aleurone grains) or crystals, which may be either dis- 



