156 PHYSIOLOGY OF NUTRITION 



Liquid cohesion, which is so important in the water system of the plant, may be 

 readily demonstrated by the method of Askenasy. By this method a mercury column 

 is caused to rise in a glass tube to a height exceeding that of the barometer column, 

 and the excess is a measure of the amount of strain developed in the water above the 

 mercury. 



5. Movement of Organic Substances. — Organic materials (such as sugars, etc.) 

 must be in aqueous solution to move from one region of the plant to another. Evi- 

 dence points to the sieve-tubes of the phloem as the main path of movement of these 

 solutes. They may diffuse, however, in all directions, so far as the protoplasmic mem- 

 branes and cell walls are permeable to them. Also, some organic materials move in 

 the transpiration stream, through the xylem vessels. 



Carbohydrates produced by photosynthesis in the cells of green leaves, in sunlight, 

 diffuse outward and move to other parts of the plant through the phloem. They, and 

 other organic materials, frequently accumulate in storage tissues, often going out of 

 solution there (e.g., starch). Such accumulations usually dissolve again later, and 

 move once more when new growth begins. The molecular physics of this movement 

 through the phloem is not understood; the rate of the movement is too great to be 

 accounted for by simple diffusion. 



Materials enter the plant body and move about therein according to the principles 

 and considerations that have been briefly stated. It remains to consider their exit 

 from the plant body. As has been shown, water is almost continually being given off 

 to the surrounding air by ordinary plants (transpiration, guttation). It has also 

 been mentioned that salts, sugars, etc., are given off to a small degree through gut- 

 tation and gland action. Oxygen passes from green leaves into the air during sunlight 

 periods, and some carbon dioxide escapes in a similar way during periods of darkness. 

 Very small amounts of volatile materials besides water, carbon dioxide, and oxygen, 

 are given off by transpiration (volatile oils recognized as odors, etc.). Salts and 

 organic substances are given off when flowers, fruits, leaves, bark fragments, etc., 

 fall away. Roots regularly give off carbon dioxide, sometimes organic acids and their 

 salts. Numerous materials are given off to the soil when roots, root hairs, and the 

 tissues of the root-caps die and decay. The amount of nitrogenous material in the 

 soil is often markedly increased in this manner by the growth and decay of legume 

 roots with tubercles. Finally, the material in the plant body is ultimately released 

 to the environment after the death of the plant, the various substances being 

 subsequently decomposed by the action of other organisms, such as bacteria, animals, 

 etc. 



