PHYSIOLOGY 467 



turgescence of the cells of which the wilted organs are composed. A 

 similar loss of turgescence may be produced by placing the tissues in 

 a solution denser than the cell-sap e.g. transferring a plant from 

 fresh to salt water. 



Osmotic pressure in the cell, indicated by its turgescence, is neces- 

 sary in order that the cell may perform its functions. The transfer of 

 most plants into a concentrated solution, as from fresh to salt water, 

 results in death, unless this is done gradually enough for the plant to 

 accustom itself to the changed conditions, and to change the concen- 

 tration of the cell-sap so as to maintain the normal osmotic pressure. 

 The maintenance of turgor may be due to the osmotically very active 

 salts of organic acids developed in the cell in the processes of nutri- 

 tion. 



Transpiration 



All aerial organs are subject to a certain loss of water by evapora- 

 tion. This loss of water is one of the most important factors in the 

 change of turgor in the cells, and is therefore of much importance 

 in regulating the movements of water within the plant. 



The gases circulate freely through the intercellular spaces, and 

 are absorbed by the cells where they are needed. Thus within the 

 green tissue, or mesophyll, of ordinary leaves, the cells are irreg- 

 ular in form, leaving large spaces between, through which the C0 2 

 circulates, to be taken up by the green cells which used it in the 

 process of photosynthesis. Where air spaces are large and commu- 

 nicate with the external atmosphere, the pressure within the plant is 

 practically the same as outside ; but in the closed cavities, like those 

 in the tracheids, the pressure may be quite different, and these differ- 

 ences in pressure must affect the movements of the gases. Certain 

 external conditions, such as changes in temperature and barometric 

 pressure, and movements of the plant body due to wind, all affect 

 more or less strongly the movements of both gases and liquids within 

 the plant. 



Special Aerating Organs. Sometimes special organs for aeration 

 are developed. Such are the lenticels upon the twigs of many woody 

 plants, and the curious outgrowths (" knees ") developed from the 

 submersed roots of the Cypress (Taxodium). 



Absorption of Water 



In vascular plants the water is generally absorbed from the earth 

 by the roots, the root-hairs being usually the direct absorbent 

 organs. There are exceptions to this, however, and some plants can 

 absorb water from the atmosphere. This is the case in Gymno- 

 gramme triangularis, a common Californian Fern, whose leaves 



