242 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



The very large amount of starch found in the leaves grown in moist air is spe- 

 cially noteworthy, as is also the observation that this high starch content is con- 

 comitant with relatively low amounts of the other substances here considered. 

 It is supposed that the carbohydrates formed in the leaves are combined, in 

 other regions of the plant, with elements derived from the soil solution; in this 

 case there was a deficiency of these elements in the plants grown in moist air, 

 owing to their low transpiration rate, so that much of the starch was retained in 

 the leaves^ This great accumulation of starch is probably one of the causes for 

 the relatively large size of leaves grown in a moist atmosphere. Differences m 

 inorganic salt content therefore constitute a second cause for the differences 

 in plant form produced by differences in the water conditions of the external 

 environment. 



Plant growth and development are markedly influenced by the concen- 

 tration of dissolved mineral salts in the surrounding medium, as has been known 

 for a long time, from studies of plant cultures in solutions of different concen- 

 trations. Plants grown in weak solutions resemble those of moist regions while 

 those grown in very strong solutions have a markedly xerophytic appearance.* 

 It is immaterial, therefore, whether the plant receives excessive amounts of 

 the minerals through high rates of transpiration or through culture in concen- 

 trated solutions, the result being the same in both cases: namely, the forma- 

 tion of short internodes, thick cell walls, etc., with generally marked tissue 

 differentiation.' 



Many strand plants have xerophytic characteristics in spite of the moist 

 surroundings in which they grow. Schimper^ noted this fact and explained it 

 teleologically by supposing that these plants, growing in sand that is frequently 

 saturated with a concentrated salt solution — from the ebb and flow of the tides 



1 Nobbe, F., and Siegert, T., Beitrage zur Pflanzencultur in wasserigen Nahrstofflosungen. I. Ueber die 

 Concentration der NahrstofHosungen. Landw. Versuchsst. 6 : 19-4S. 1864. [For a thorough review of 

 the literature of water-cultures, see: Tottingham^ 1914. (See note d, p. 78.)] 



' Schimper, A. F. W., Ueber Schutzmittel des Laubes gegen Transpiration, besonders in der Flora Java's, 

 Sitzungsber. K. Preuss. Akad. Wiss. Berlin 1890 : 104S-1062. 1890. 



• Whether these responses have any relation to the supply of mineral salts is at least ques- 

 tionable. The phenomena here dealt with in a very cursory way are exceedingly complex 

 and cannot be generally and satisfactorily explained along the lines followed by the author. 

 The water content of the tissues appears, in itself, to act as the main control in such cases as 

 are here brought forward. It ought to be remarked that tliis water content of the plant, or of 

 any tissue, is a function of the relation that has previously obtained between the rates of 

 water entrance and of water exit. Concentrated solutions about the roots retard water 

 entrance in much the same way as does a soil of low moisture content. The last sentence 

 in the text might be reasonably replaced by the following one: It is immaterial, therefore, 

 whether the water content of the plant becomes low through high rates of water loss or 

 through low rates of water intake. — For discussions of some of the considerations that are 

 not clearly set forth in the text but are quite necessary in dealing with this general subject of 

 plant water relations, see: Livingston, B. E., and Hawkins, Lon A., The water-relation be- 

 tween plant and soil. Carnegie Inst. Wash. Pub. 204: 3-48. 1915. Ptilling, H. E., and 

 Livingston, B. E., The water-supplying power of the soil as indicated by osmometers. IM. 

 204 : 49-84. i9rs. These papers furnish numerous other references to the literature. — Ed, 



