CHIEF ENVIRONMENTAL CONDITIONS. 113 



This water-loss, constituting stoma tal transpiration, usually occurs 

 at a rate much higher than that evidenced by cuticular transpiration. 

 The ubiquity of this form of water requirement and its generally 

 great magnitude render it the dominating physiological moisture 

 condition for most plant-forms growing in the open air. 



However it may be removed, the water-content of an active tissue 

 that is being depleted is always replenished at a greater or less rate 

 from other tissues in the vicinity or from without the body. If such 

 renewal of the water-content fails for a considerable time, partial or 

 complete loss of activity must result. In plants without storage- 

 organs the usual transpiration-rate can continue but a short time 

 without entrance of water from the outside. Where storage-tissues 

 are present the external supply may, of course, be cut off for a longer 

 time. 



Since transpiration is the most important factor in determining the 

 need for an external water-supply to the plant, it will be necessary to 

 consider here some of the conditions that influence this process. Its 

 rate is dependent upon three conditions: (1) the structure and condi- 

 tion of the leaves or other transpiring parts ; (2) the evaporating power 

 of the air; (3) the intensity and quality of illumination. As is well 

 known, there occur in different plants, under identical external condi- 

 tions, great differences in the rate of transpiration per unit of surface. 

 Certain structures, such as waxy and hairy coverings, palisade tissue, 

 and other features, make cuticular transpiration markedly less pro- 

 nounced than it might otherwise be, and many stomatal characters 

 similarly influence the rate of stomatal water-loss. Many anatomical 

 characters are known to become permanently altered by age, of course 

 with reference to the nature of the environmental complex, as where 

 cuticular thickening may increase or fail to do so, according to the 

 age and surroundings of the plant. Such physiological responses are 

 apparently dependent upon the rate of movement of transpiration- 

 water (Pfeffer-Ewart, Plant Physiology, Z: 121). In regions where 

 the evaporating power of the air rises rapidly during the growing- 

 season, plants with a highly developed cuticular response of the form 

 just mentioned may be expected to survive longer than others with a 

 less-marked response of this kind. Stomatal movements, of opening 

 and closing, may also be of considerable importance in certain cases, 

 though this whole question is sadly in need of a more thoroughgoing 

 physical investigation than it has yet received.^ 



* Concerning the quantitative aspect of differences in the power of plant leaves to retain 

 water and thus retard water-loss, see the following papers: Livingston, B. E., The relation of 

 desert plants to soil-moisture and to evaporation, Carnegie Inst. Wash. Pub. 50, 1906. — • 

 Idem, The resistance offered by leaves to tran.spirational water-loss. Plant World 16: 

 1-35, 1913. — Livingston, B. E., and A. H. Estabrook, Observations on the degree of stomatal 

 movement in certain plants. Bull. Torr. Bot. Club 39: 15-22, 1912.— Bakke, A. L., Studies on the 

 transpiring power of plants as indicated by the method of standardized hygrometric paper. 

 Jour. Ecol. 2: 145-173, 1914. 



