24 WATER-RELATION BETWEEN PLANT AND SOIL. 



conditions almost nothing is known, excepting that they are often appar- 

 ently very important. 



It may not be out of place here to consider a little more deeply the 

 general nature and relations of these quantitatively unknown internal 

 conditions. We may begin with the aerial periphery of the plant 

 and progress downward. Transpiring power, which has now been 

 studied to some extent, resides only at the surfaces of exposed mem- 

 branes of leaves, etc. It is commonly granted to vary with the extent 

 of these membranes, which extent varies with the increase and decrease 

 of general transpiring surface (as with the formation of new leaves and 

 the falling away of old ones). It also varies with the effective exposure 

 of the moist membranes of any single leaf during any single day, 

 which fluctuates with stomatal movements wherever these occur; the 

 closing of each stoma removes from effective contact with the exterior 

 a certain amount of moist cell-wall surface. Beyond this, transpiring 

 power depends upon the attraction exerted between the gel-colloidal 

 walls of the transpiring cells and their imbibed water. This attraction 

 is clearly a function of the nature of the wall and of its water content ; 

 with a given wall, the more nearly it approaches saturation with water 

 the more nearly must its transpiring power approach a maximum. 

 The sort of fluctuation suggested by the last clause has been empha- 

 sized by Livingston, Renner, Caldwell,and Edith B. Shreve as incipient 

 drying or saturation deficit, which appears to be a condition in the 

 control of certain important fluctuations in transpiring power. It is 

 more commonly emphasized, at least in a general way, that alteration in 

 the nature of exposed cell walls is frequently accompanied by profound 

 changes in transpiring power; the power of foliar epidermis, for example, 

 to give off water vapor, is well known to vary through cutinization and 

 through impregnation with wax-like and other non-water-attracting 

 substances. Foliar surfaces often alter markedly in this way, as leaves 

 pass through progressive developmental phases. Much more is known 

 of the sequence and nature of these wall changes, however, than of 

 their effects upon transpiring power. 1 Such changes are usually per- 

 manent and not readily reversible, unlike stomatal changes and the 

 phenomena of incipient drying. 



'These changes in the nature of cell walls are apparently more or less controlled by internal 

 conditions, sequences of metabolic processes and all the maze of as yet uninvestigated conditions 

 suggested by the words ontogeny and development. But they seem to be frequently determined, 

 in large measure, by external conditions, the whole problem being made very complex by the fact, 

 for instance, that transpiration rate appears to condition cutinization, while the degree of cutini- 

 zation is an important condition controlling transpiration. It is such retroactive causal relations 

 that make natural phenomena appear purposeful, to many minds, until they have been analyzed 

 to a degree. Pfeffer made a remark which suggests problems for much constructive study in 

 relation to transpiring power when he wrote: "The influence of transpiration in favoring the 

 development of the cuticle and of the conducting channels seems to be directly connected with the 

 movements of water it induces." (Pfeffer, W., Physiology of plants, translated by A. J. Ewart 2 : 

 121. Oxford. 1903.) This apparent direct connection may perhaps come about through the 

 phenomenon of incipient drying. 



