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-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, 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. 



