196 PHYSIOLOGY OF NUTRITION 



their formation; sugar must be plentifully supplied and, in some cases at least (as 

 in some roots), oxygen must not be too plentiful. 



11 [7]. Alkaloids, Toxins, Antitoxins. — A very large number of different kinds of 

 substances, some of which are very poisonous, are formed in plants. Here may be 

 mentioned solanin, an alkaloid that is formed in the potato, especially in wounded or 

 actively growing regions. Bacteria form poisonous substances called toxins, which 

 diffuse out of the cells into the surroundings. Some bacteria are parasitic and develop 

 in the bodies of living higher plants and animals, these being the pathogenic forms. 

 Pathogenic, as well as other, bacteria are generally grouped as aerobic or anaerobic, 

 accordingly as they require free oxygen or are poisoned by it. Some forms (the faculta- 

 tive anaerobes) can live without free oxygen but are not injured by its presence. 



The tetanus bacillus is an anaerobic form. Its development in the human body is 

 confined to the neighborhood of the infected wound, but the very active toxin that it 

 produces spreads throughout the body of the victim, causing death by lock-jaw. The 

 anthrax bacillus, which causes splenic fever of cattle, etc., is aerobic and can be grown 

 in bouillon. It was through Pasteur's studies of this organism that he discovered the 

 bacterial nature of infectious diseases. He also laid the foundations of serum therapy, 

 vaccination for immunization, etc., from experiments with anthrax bacillus. The 

 peculiar toxin emanating from the cells of the parasitic bacillus stimulates the cells of 

 the diseased animal to give off, into the blood-stream, a specific antitoxin. This tends 

 to counteract the poisonous influence of the toxin, and if sufficient antitoxin can be 

 formed soon enough (or is supplied from the blood serum of another animal that has 

 been immunized by previous vaccination with a weakened strain of the parasite), the 

 disease is cured. Diphtheria and tetanus are treated in this way. The solution 

 obtained by filtering (through a Chamberland filter) a bouillon culture of the 

 diphtheria bacillus is very poisonous because of the toxin that is present. 



12. Water.— Physiological processes cannot go on without a plentiful supply of 

 water; the chemistry of life is almost entirely that of aqueous solution. Active plant 

 tissues contain 80 or 90 per cent, (by weight) of water. Resting cells (as in seeds) 

 become very dry, but they must be freely supplied with water before they become 

 active. The water-soluble substances of the organism are largely dissolved in the water 

 of active cells, and many insoluble materials swell in water until the colloid dispersion 

 formed has many of the properties of a true solution. Electrolytes become ionized 

 (and consequently more active chemically) when in aqueous solution. Furthermore, 

 water itself enters into numerous chemical reactions in living cells; for example, the 

 photosynthesis of carbohydrates, and the hydrolytic decompositions of carbohydrates, 

 proteins, etc., by enzymes. Practically all of the hydrogen of the plant is derived 

 from water. Some water is produced in living cells, through respiration, through the 

 formation of the complex carbohydrates, proteins, etc., from simpler substances, 

 but by far the greater part of the water in a plant has entered as such, from the soil. 

 Much water escapes from ordinary plants by transpiration, some by guttation, glandu- 

 lar secretion, etc. 



13. Germination of Seeds. — The resting seed contains all the substances needed 

 for a considerable amount of growth, excepting water and oxygen; the latter substances 

 are absordeb as germination starts and proceeds. The earlier stages of germination, 

 until the photosynthesis of carbohydrates becomes pronounced, result in the using up 

 of the non-aqueous materials of the seed; thus the dry weight of a young seedling is 

 smaller than that of the ungerminated seed. 



