IV. 



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 microorganisms 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- 



