SECRETIONS 281 
combined with nitrogen to form amino-compounds or amides, 
which then combine with themselves and other mineral elements 
to form proteins. The proteins, like the starches and celluloses, 
are thus supposed to be built up by the combining of simpler 
compounds. That amino-compounds are involved in the for- 
mation of proteins is suggested by the fact that they are nearly 
always present in plants, and also by the fact that they are pro- 
duced when proteins are decomposed. Asparagin CyHsO3N, is 
one of the most common amino-compounds found in plants, but 
a number of others, such as arginin, tyrosin, leucin, and trypto- 
phane, are often found in considerable quantities in the germinat- 
ing seeds or seedlings of plants. 
Plants form many kinds of proteins, but most of them belong 
to one of the general classes — albumins, globulins, glutelins, 
guiadins, or nucleo-protens. The albumins, the proteins of 
which the white of an egg is composed, are represented in Peas 
by legumelin and in Wheat and other cereals by leucosin. The 
globulins are common in the Legumes, legumin being the chief 
one. The globulins are probably the most abundant of the re- 
serve proteins in all seeds except cereals. Glutenin found in Wheat 
and oryzenin in Rice belong to the glutelins. The gliadins are 
found in the cereals. Gluten, the substance upon which the tenac- 
ity of dough depends, consists chiefly of glutelins and gliadins. 
The nucleoproteins occur in the nucleus of the cell where they 
are an important constituent of chromatin. Most proteins are 
insoluble in the cell sap, and need to be digested by enzymes into 
soluble and diffusible forms, such as proteoses, peptones, or amino- 
compounds, before they can be moved through the plant. 
Secretions. — A large number of plant substances, differing 
widely in both composition and function, are often classed as 
secretions. The important classes of secretions are volatile oils, 
glucosides, alkaloids, pigments, and enzymes. Some of the 
secretions accumulate in glands, which have special cells for 
secreting and often cavities provided for holding the secretions, 
as the glands on the leaves of the Mints and in the skin of Oranges 
illustrate. Some accumulate in long ducts like those in the stems 
and leaves of the Milkweeds and Pines and are secreted by the 
cells around these ducts. Many of the secretions, however, are 
formed in cells, in which secreting is only a minor function, and 
are usually found in solution in the vacuoles of the protoplasm. 
