TV. 
PRODUCTS OF VITAL ACTIVITY. 
ALL living cells, animal or vegetable, while in active growth, 
appropriate certain elements for their nutrition from the pabulum 
with which they are supplied, and at the same time excrete certain 
products which, in some cases at least, it is their special function to 
produce. In the higher plants and animals specialized cells excrete 
substances which are injurious to the economy of the individual, 
and secrete substances which are required to maintain its existence. 
As an example in animals we may mention the excretion of urea by 
‘the epithelium of the kidneys, the retention of which is fatal to the 
individual, and the gastric secretion which is essential for its con- 
tinued existence. Among the higher plants we have an immense 
variety of substances formed in the cell laboratories, some of which 
are evidently useful for the preservation of the species, while others 
are perhaps to be considered simply as excretory products. The 
odorous volatile products given off by flowers are supposed to be 
useful to the plant in attracting insects by which cross-fertilization 
is effected. The various poisonous substances stored up in leaves 
and bark may serve to protect the plant from enemies, etc. 
The minute plants with which we are especially concerned also 
produce a great variety of substances, some of which may be useful 
to the species in the struggle for existence. Thus the deadly pto- 
maines produced by some of the pathogenic bacteria serve to para- 
lyze the vital resisting power of living animals and enable the para- 
sitic invader to thrive at the expense of its host. In the present 
section we shall consider in a general way these various products of 
bacterial growth. 
Pigment Production.—A considerable number of species are 
distinguished by the. formation of pigment of various colors and 
shades. We have all of the shades of the spectrum from violet to 
red. The color, as a rule, is only produced in the presence of oxy- 
gen, and when the pigment-producing microérganisms are massed 
upon the surface of a solid culture medium the pigment production 
is often limited to the superficial portion of the mass. In some 
cases a soluble pigment is formed which is absorbed by the transpa- 
