TRANSPIRATION 177 



tion (usually the roots), to other parts of the plant. This move- 

 ment of water is not well understood but is no doubt due to many 

 factors, of which the most important are: (1) the movement 

 from living cell to living cell by the laws of osmosis, (2) the 

 movement from cell to cell by the laws of diffusion, and (3) the 

 movement through the fibrous, cells, more especially the tracheary 

 tubes, by the laws of capillarity. However, the combination of 

 these forces does not appear fully to explain the movement of 

 the water from the roots to the tops of tall trees. 



When the water reaches the leaves it is transmitted to the 

 mesophyll and palisade cells. Some of it is used in the forma- 

 tion of carbohydrates (see page 195) and possibly other com- 

 pounds, but by far the greater part passes through the cell walls 

 into the intercellular spaces and out through the stomata. Since 

 the amount of water in the leaf is being reduced by transpiration, 

 there must be a continuous movement of water from the roots, 

 up through the stems and into the leaves. Of course, this moving 

 water carries the various compounds which are in solution in it. 

 Transpiration is a continuous process, varying in accordance 

 with the age of the organs, the part, of the plant, the season of 

 the year, the hour of the day or night, the humidity, the tem- 

 perature and many other factors. One of the most important 

 secondary effects is the lowering of the temperature of 

 the plant, thereby preventing injury due to excessive heat. 



Excessive Transpiration. The results of excessive tran- 

 spiration are pronounced and frequently the cause of heavy crop 

 losses. It checks the growth and yield of many plants, espe- 

 cially corn ; it prevents pollination in some plants and causes the 

 falling of flowers and leaves and sometimes twigs in others. The 

 falling of twigs, frequently referred to as self-pruning, is very 

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