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- 
