238 VEGETABLE PHYSIOLOGY 



cells. They have been called gliadin and glutenin ; 

 occurring separately in the seed, they interact with one 

 another in the presence of water and form the gluten of the 

 flour. Like the zein of maize, these proteins belong to the 

 peculiar class whose members are soluble in dilute alcohol. 



In many cases the proteins of the reservoirs do not 

 remain unchanged during the resting period which follows 

 their deposition. This is especially the case with seeds, in 

 which such changes are characteristic of the process known 

 as ripening. 



Proteins occur also in the temporary reservoirs to 

 which allusion has been made. Fleshy roots and stems 

 contain them in amorphous form in their parenchyma ; 

 certain forms are met with in the sieve tubes, and are 

 coagulable on boiling like the globulins of the seeds. The 

 proteins which are constant constituents of latex are no 

 doubt in great part reserve food- stuffs. 



In many cases amido-acids such as asparagin may 

 be detected in the sap of various cells. These may be 

 reserve materials temporarily retained where they are 

 found, or they may be only translocatory products. Their 

 occurrence in some resting seeds suggests the former 

 explanation of their presence. It is not easy to detect 

 them in the cells, as they are dissolved in the sap, but in 

 many cases they can be caused to crystallise by placing a 

 section of the tissue on a glass slip in glycerine. 



A great many plants store quantities of complex sub- 

 stances known as glucosides. These are bodies which on 

 decomposition give rise to some kind of sugar and some 

 other product or products, usually belonging to the 

 aromatic series of carbon compounds. Among them may 

 be mentioned amygdalin, which is found in the seed of the 

 bitter almond. During germination it splits up into 

 benzoyl aldehyde, hydrocyanic (prussic) .acid, and grape- 

 sugar. Many such bodies are known, and they are some- 

 what widely distributed. Some occur in seeds, but they 

 are more frequently represented in the reservoirs con- 



