I08 PHYSIOLOGY. 



cose by a diastatic " ferment " formed by the seedling as it germinates. 

 Make a thin cross-seetion of a grain of wheat, incKiding the seed coat and a 

 portion of the interior, treat with iodine and mount for microscopic exam- 

 ination. Note the abundance of starch in the internal portion of endo- 

 sperm. Note a layer of cells on the outside of the starch portions filled 

 with small bocUes which stain yellow. These are aleurone grains. The 

 cellulose in the cell walls of the endosperm is dissolved by another enzyme 

 called cytase, and some plants store up cellulose for food. For example, in 

 the endosperm of the date the cell walls are very much thickened and pitted. 

 The cell walls consist of reserve cellulose and the seedling makes use of it 

 for food during growth. 



216(;. Albuminous and exalbuminous seeds. — In seeds where the food is 

 stored outside of the embryo they are called albuminous; examples, corn, 

 wheat and other cereals, Indian turnip, etc. In those seeds where the food 

 is stored up in the embryo they are called exalhuiiiinous; examples, bean, 

 pea, pumpkin, squash, etc. 



217. Digestion has a well-defined meaning in animal physiology and 

 relates to the conversi(jn of solid food, usually within the stomach, into a 

 soluble form by the action of certain gastric juices, so that the li(|uid food 

 may be absorbed into the circulatory system. The term is not often ap- 

 plied in plant physiology, since the method of oljtaining food is in general 

 fundamentally different in plants and animals. It is usually applied to 

 the process of the conversion of starch into some form of sugar in solution, 

 as glucose, etc. This we have found takes place in the leaf, especially at 

 night, through the action of a chastatic ferment developed more abundantly 

 in darkness. As a result, the starch formed during the day in the leaves is 

 digested at night and converted into sugar, in which form it is transferred 

 to the growing parts to be employed in the making of new tissues, or it is 

 stored for future use; in other cases it unites with certain inorganic sub- 

 stances, absorbed by the roots and raised to the leaf, to form proteids and 

 other organic substances. In tubers, seeds, parts of stems or leaves where 

 starch is stored, it must first be "digested" by the action of some enzyme 

 before it can be used as food by the sprouting tubers or germinating seeds. 



For example, starch is converted to a glucose by the action of a diastase. 

 Cellulose is converted to a glucose by cytase. Albuminoitls are converted 

 into available food by a tryptic ferment. Fatty oils are converted into 

 glucose and other products by lipase. 



Inulin, a carbohydrate closely related to starch, is stored up for food in 

 solution in many composite plants, as in the artichoke, the root tuber of 

 dahlia, etc. When used for food by tlie gnnviiig plant it is converted into 

 glucose by an enzyme, inulase. Make a seelion of a portion of a dahlia 

 root and immerse in Os'^o aUohol for sr\'eral hnurs. The inulin is preeijii- 

 tated into spha.-ro crystals. (.See also paragrajths i:;6-iOi and 21O6.) 



