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 ceU-walls, 

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

 stone. Nitrogenous food-reserves are found chiefly ia 

 seeds, but, unlike carbohydrates, they 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 

 ..d economic purposes. 



1. The Seed.— The food- 

 material stored up ia 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.-LONG1TTOINAL Section it is unable adequately to 

 or ALBTJMtNOiis Seed or Onion. i -j ^ j "•-■ 



(Masnified.) supply its own needs. In 



a, Beed-coat ; 6, endosperm ; c, embryo, dlhuminous 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 aU 

 the Umbelliferse. In exalhuminous 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, sun- 

 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-., some varieties of 

 maize. 



(iii.) Cellulose — e.g., date and other palm seeds. 



(6) Protein occurs in the form of amorphous particles" 



