CHIEF ENVIRONMENTAL CONDITIONS. 115 



is not at all understood, but we may be sure that the water-loss thus 

 occasioned is of no great general importance in the distribution of 

 plant-forms, especially since the greatest excretion of water, as in 

 guttation, usually occurs at times when the transpiration-rate is low 

 and the supply of water within the tissues is relatively great. In- 

 deed, one of the common teleological conceptions bearing upon the 

 so-called regulation of water-loss by plants is that these organisms 

 actively and purposefully force water out of theu- tissues whenever 

 they have been compelled by external circumstances to absorb more 

 of the liquid than they want. 



To summarize the preceding paragraphs, the entrance of water into 

 the ordinary active plant is essential to its activity: (1) because it is 

 necessary for the enlargement of water-saturated cells; (2) because it 

 is destroyed in photosynthesis, etc.; (3) because it is continually being 

 lost by cuticular and stomatal transpiration and by excretion. 



2. SUPPLY OF WATER TO THE PLANT. 



As has been emphasized above, the transpiring tissues of a plant 

 must receive water from elsewhere, otherwise they would soon become 

 wilted and collapse, and other active tissues must likewise receive 

 water, though frequently in much smaller amounts. There are 

 several sources from which this water may come. Water-storage 

 tissues and dying cells, or cells passing into a dormant stage, may 

 furnish more or less water to other tissues, according to the form of 

 plant considered and its phase of development. A relatively very 

 small amount of water, fixed previously by photosynthesis, must be 

 set free in the tissues by the activity of respiration, and this may 

 become available for growing cells or may be again transformed by 

 photosynthesis. Also, the reverse of hydrolytic decomposition (such 

 as the synthesis of starch from glucose) results in the chemical formation 

 of some water. For the transpiration of ordinary plants the latter 

 sources are surely inadequate.-^ 



During rains, and when the temperature of the foliage falls below 

 the dew-point of the surrounding air, the external surfaces of the 

 plant become wet, and a considerable amount of moisture may enter 

 the plant-body through the cuticle and even through stomatal open- 

 ings. This source of water is especially important only for certain 

 forms, such as mosses, liverworts, and plants of similar water-rela- 

 tions. With heavy cuticle, trichome coverings, etc., very little water 



^ Fitting suggests that respiration water may be important in this connection in certain desert 

 tubers, etc. (See Fitting, Hans, Die Wasserversorgung und die osmostischen Druckverhaltnisse 

 der Wtistenpflanzen, Zeitschr. Bot. 3: 209-275, 1911. — Livingston, B. E., The relation of the 

 osmotic pressure of the cell-sap in plants to arid habitats, Plant World 14: 153-164, 1911.) 

 The latter is in part a review of Fitting's paper. An excellent discussion and the most valuable ex- 

 perimental treatment yet available of the amount and importance of respiration water in plants 

 and animals is the following: Babcock, S. M., Metabolic water: its production and role in vital 

 phenomena, Wisconsin Agric. Exp. Sta. Bull. 22, 1912. 



