128 PLANT SOCIOLOGY 



leaf to leaf on the same plant. In woody plants, short shoots transpire 

 more than long shoots, and one-year-old shoots more than two year 

 old (Ivanov, 1924). Hence calculations of the total amount of tran- 

 spiration are extremely difficult. Attempts to deduce a measure of 

 xerophytism from the transpiring power of plants (c/. Bakke, 1914; 

 Pool, 1923) have yielded no very satisfactory results. 



From observations of Walter (1925) it appears that a unit surface 

 of foliage is not significant as a measure for comparison of amounts of 

 transpiration. For, if the transpiration per unit of surface is measured 

 for larger and smaller leaves, the smaller leaves show a greater amount. 



Root Suction and Osmotic Concentration. — Fitting (1911) first 

 called attention to the absence of xeromorphic adaptation in certain 

 north African steppe and desert plants such as Peganum harmala. 

 These mesomorphic species withstand the longer summer drought 

 without forming protective devices of morphoanatomical nature. The 

 absence of drought-resisting mechanisms in these plants is made up for 

 by a high suction power. Poma (1922) showed that when the osmotic 

 concentration of the culture solution increases, the suction tension of 

 the roots, that is, the intracellular osmotic pressure, constantly 

 increases. 



Table 7. — Increase in the Suction Tension (in Atmospheres) of Roots of 



Species of the Juncetum maritimi, Following the Increased Osmotic 



Concentration of the Culture Medium 



(After Poma) 



Osmotic concentration of tlie culture solution 



6 11 



22 



Triglochin maritimum . 



(Hycerin maritima 



Jzmcus maritirnus 



10 

 10 



7 



20 

 23 

 20 



24 

 27 

 23 



30 

 34 



The daily curve of suction tension runs parallel to the curve of 

 saturation deficit of the air. It rises with increasing deficit (decreasing 

 humidity) up to midday, then falls until the following morning. The 

 fluctuations of humidity correspond to fluctuations in the suction power 

 in the plant. Table 8 gives measurements of these fluctuations. 



The highest suction tension was therefore found in rainless periods 

 on days of great variation in humidity. Ursprung discovered the 

 interesting fact that when water absorption by roots is hindered by 

 frozen soil, an increase in osmotic pressure follows just as in desert 

 plants. The suction force of expressed sap of t he perianth of Hellehorus 

 rose to 50 atmospheres in winter as against 10 atmospheres in May. 



