244 VEGETABLE PHYSIOLOGY 
The seeds of the cereal grasses contain two other very 
curious reserve proteins, which give rise in the flour to a 
peculiar sticky material which is generally known as gluten. 
They do not appear to be present in the aleurone grains 
of the seeds, but to occur in the starch-containing 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-stufts. 
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, together with 
other products usually belonging to the aromatic series 
of carbon compounds. Among them may be mentioned 
amygdalin, which is found in the seeds of the bitter almond. 
