NUTRITION, STORAGE, AND RESPIRATION 



J 09 



appears most frequently in the form of Starch-Grains (Fig. ^^) ; 

 occasionally it takes the form of a cellulose thickening of the cell-walls, 

 as in the Date stone and the seeds of other Palms (Fig. 78). These amy- 

 loid substances being insoluble are a compact form of storage. Carbo- 

 hydrates may also be laid aside as Sugar (Sugar-Cane, Beet, Onion), 

 or Inulin (Dahlia roots, Artichoke tubers), which being soluble sub- 

 stances are less compact, and are invisible in the living tissues ; but 

 either of them may be precipitated as 

 crystals on extracting the water from the 

 transparent tissues by alcohol. The Fats 

 are commonly present in the form of large 

 or small oil-globules in the protoplasmic 

 body. Oil may also be distributed 

 throughout the protoplasm, giving it a 

 highly refractive appearance. Since 

 carbohydrates are usually absent where 

 oils are thus stored in quantity, it appears 

 that the latter may serve physiologically 

 as substitutes for the former. A good 

 example is seen in the endosperm of the 

 Castor-Oil seed, where starch is absent 

 though it makes its appearance in the 

 seedhng. Oil is a very compact form of 

 storage of combined Carbon. It has a 

 higher percentage of Carbon than carbo- 

 hydrates. Accordingly it is not un- 

 commonly found in small seeds, where 

 compactness of storage is essential. 

 Proteids are frequently stored in the 

 form of the dense protoplasts of the cells, 

 as in the endosperm of Palms. But they may take definite form as 

 Aleurone Grains, or Crystalloids. The latter are seen in the Potato 

 (Fig. 79, C), where they have a very regular cubical form. Aleurone 

 Grains are found in seeds, and are especially large where oil is also 

 stored : as for instance, in the Castor-Oil seed, where they are oval in 

 form, and contain within an amorphous outer proteid coat a crystalloid 

 and a globoid composed of phosphates (Fig. 80). Such are the forms 

 in which excess of nutritive materials may be stored. The chief 

 interest lies in the insoluble forms of storage. Their deposit involves 

 as a rule chemical change. For instance, sugar which is soluble and 

 can readily be transferred by diffusion, may be changed into insoluble. 



Fig, 78. 

 Cell-wall of a single cell of Ihe endo- 

 sperm of Lodoicea, consisting of storage- 

 cellulose which forms the thickened 

 regions of the wall. ( .^oo.) (After 

 Gardiner.) 



