274 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



Unequal amounts of moisture on the two opposite sides of a plant organ also 

 exert an influence upon growth. If seeds germinate in a sieve filled with saw- 

 dust and suspended so that its bottom is at an angle of 45 degrees from the 

 horizontal (Fig. 124), the primary roots soon penetrate through the openings 

 in the bottom, but they grow no farther in the vertical direction. They bend 

 laterally toward the bottom of the sieve and grow downward along its outer 

 surface, to which they become closely appressed. This bending of plant organs 

 toward water, or away from the drier side, is called positive hydrotropism. 



§5. Dependence of Growth and Configuration upon Light. 1 — Light exerts a 

 marked influence upon the rate of plant growth as well as upon the formation 



Fig. 124. — Experiment showing positive hydrotropism of roots. 



answer to this question seems to be, soil conditions that hinder water absorption. Toxic sub- 

 stances appear to do this by poisoning the roots, so that these organs possess but a limited 

 power to take up water, in spite of the presence of a plentiful supply of water in the soil. Low 

 soil temperature (as in subarctic regions) may hinder water absorption from wet soils in some- 

 what the same way. These considerations may furnish at least a partial explanation of the 

 fact (if it be a fact) that plant forms without special foliar structures that retard water loss are 

 generally unable to thrive in bogs and in the far north. 



The ecological question just touched upon is one with which physiology, as such, need not 

 be concerned, and distributional and physiological problems ought not to be so commonly 

 confused as is now the case in botanical literature. Physiology enquires how the plant comes 

 to be what it is, and how it operates as a machine. Its explanations have to deal with migra- 

 tions and transformations of matter and energy. Distributional ecology, on the other hand, 

 enquires what are the characteristics of any plant form and of any given set of environmental 

 conditions, by virtue of which the given habitat can or cannot support the plant form con- 

 sidered, or by virtue of which the plant form can or cannot thrive in the given habitat. How 

 the conditions of the habitat came to be what they are, involves questions of climatology, 

 physiography, soil science, etc.; why the plant has the internal characteristics that it has, 

 involves questions of physiology. — Ed. 



1 Wiesner, J., Der. Lichtgenuss der Pflanzen. Leipzig, 1907 Idem, same title. Verha ndl. Ges. 

 Deutsch. Naturforscher u. Aerzte 81: 66-86. 1909. 



