PHYSIOLOGICAL 317 



the cell, or may become compacted into grains, as in beans and 

 wheat. Storage of proteins is almost unknown in animals. Amino- 

 acids are exceedingly common in plant stores, usually along with 

 other reserves. A rare but interesting condition is illustrated by 

 some condensed alkaloids, such as those of "cocoa" and coffee. 



We have so far thought of most if not all the alkaloids and kindred 

 complex products, such as thein or caffein, etc., as put out of the 

 plant's way, like the oxalic acid crystals of a rhubarb leaf, rather 

 than as stores proper. Yet a strong argument has been brought for 

 also considering at least the above named in the simpler way, as not 

 without more or less reserve value. 



It is probable that the complex milk or latex (at once solution, 

 emulsion, and suspension) of poppies, india-rubber tree, etc., is also 

 in part at least of storage value. 



DIGESTION. — From storage it is convenient to pass to digestion, 

 the fermentation which changes the reserve material into mobile 

 form suitable for transport. Just as in animals, the food is made 

 soluble; and that means the splitting of a complex molecule into 

 two or more simpler ones, often with the incorporation of water. 

 Thus cane sugar, when digested in the plant, is changed into glucose 

 and fructose; and starch first into maltose, and then into glucose. 

 Though plants have no special digestive cavity, the process is funda- 

 mentally the same, and is accomplished by ferments or enzymes 

 (see Fermentation). It is important to note that some enzymes can 

 work both ways, building up as well as breaking down; thus starch 

 may be formed from glucose, cane sugar from grape sugar and fruit 

 sugar, and proteins from amino-acids. As tjrpical examples of 

 digestive ferments in plants, may be mentioned three which are 

 familiar in animals also : the diastase that changes starch into sugar, 

 the lipase that changes fats into fatty acids and glycerine, and the 

 peptic and tryptic enzymes which break down proteins. The assimila- 

 tion of these by the protoplasm of the plant-cell seems much as in 

 the animal. 



RESPIRATION. — If numerous sprouting seeds or opening flowers 

 are crowded into a corked jar, the air becomes in a few hours laden 

 with carbon dioxide. This is at once shown by the familiar tests 

 of the extinction of a lighted taper thrust into the jar, or by the 

 rapid clouding of lime-water shaken up in the fouled air. Between 

 the still living plants and the air in the jar there has been an exchange 

 of gases; oxygen is absorbed and carbon dioxide is given off. This 

 significant fact in the function of respiration has too much been 

 taken for the whole process, but that has been shown to be a too 

 superficial conception. 

 That respiration means more than exchange of gases is evident 



