104 PLANT PHYSIOLOGY 



ide, oxygen, and free nitrogen ( when the last is used at all ) 

 — ^take place in accordance with the simple and generally 

 known laws governing the diffusion of gases. We must, 

 however, bear distinctly in mind that these gases are in 

 two states in the bodies of all higher and of many lower 

 plants. Through the stomata gases pass in the gaseous 

 condition into or from the intercellular spaces, the air pas- 

 sages, etc. This movement through the stomata and in the 

 intercellular spaces is strictly by diffusion, except where 

 affected by mechanical forces such as compress or expand 

 the air-spaces, etc., etc. But when gases pass through the 

 cell-waU, into or out from a cell, their molecules mix with 

 the water-molecules in cell-wall, protoplasm, and vacuoles, 

 becoming dissolved in the water which the living body con- 

 tains as an essential part of its structure. The movements 

 of gases into and out from living cells, and from cell to cell, 

 are therefore the movements of solutes (dissolved sub- 

 stances ) . The absorption of solutions into living cells, and 

 their transfer from cell to cell, take place in accordance 

 with the laws governing the diffusion of liquids. We can, 

 therefore, study the movements of solids and of gases at the 

 same time, for, so far as living cells are concerned, these 

 two classes of substances behave alike. So far as the supply 

 of solids and of gases to the living cells is concerned, we 

 have to deal with different phenomena, and these we must 

 study separately. 



The water in the soil, and consequently all flowing water 

 and that in pools, ponds, lakes, and in the sea, is a dilute 

 solution of nutrient and other soluble substances from the 

 air, from the mineral matters of the soil, and from the 

 mixture of organic and inorganic matters collectively termed 

 humus, which is found in all but the most sterile soil. The 

 water of streams, ponds, lakes, and of the sea, is in the 

 hydrostatic or massive state. After heavy rain, flood, or 

 the melting of snow and ice, water is in the hydrostatic 

 state in the soil also. Water in this condition can be 

 drained off, but much will remain in other conditions, the 

 amount depending upon the character of the soil. Soil 

 which has been thoroughly drained, but not dried, will feel 



