206 PLANT PHYSIOLOGY 



the changes induced by irritation, whether these changes in 

 direction are first effected by changes in the tissue-tensions 

 of the motor zone or by growth itself. 



What has been said about the irritability and means of 

 response of roots which grow vertically ( orthotropic ) , ap- 

 plies equally to lateral roots which grow obliquely ( plagio- 

 tropic). The difference between these organs lies in their 

 different relations to irritation by gravity. They may not 

 differ in sensitiveness, but they will respond in different 

 degrees to the same force. We may conceive this to be due 

 to the different balances of all the influences to which the 

 two kinds of roots are subject. If both lateral and pri- 

 mary roots grew vertically, they would interfere w r ith one 

 another in position and in functions, failing to attain their 

 utmost usefulness and irritating one another by their prod- 

 ucts. This may be the main factor in determining the po- 

 sition of lateral in relation to primary roots. The response 

 of lateral roots to the stimulus of gravity will be changed 

 when the primary root is removed or so injured that its 

 further vertical growth is impossible. The lateral roots will 

 then bend down and one or more will attempt to take the 

 place in position, direction of growth, etc., of the primary 

 root. The relation of any organ to any one influence or 

 combination of influences depends upon the relation of all 

 the other organs to each one of the influences composing 

 the total to which the organism is subject.* 



Among stems as among roots there are erect (ortho- 

 tropic) and horizontal ( plagiotropic ) organs, differing 

 from one another, not in their sensitiveness or in the effi- 

 ciency of their response to gravity, but in their relation to 

 the force. In the ordinary orthotropic stem, unlike the root, 

 there is no separation into sensory and motor zones, though 

 Francis Darwin f has shown, in the seedlings of certain 

 grasses, that the first leaf-sheath is sensitive to gravitation 

 and the bending is accomplished only by the segment of 



* Consult Czapek, loc. cit., and Schober, Das Verhalten der Neben- 

 wurzeln in der verticalen Lage. Bot. Zeitung, 1898. 



f Darwin, F. On geotropism and the localization of the sensitive re- 

 gion. Annal* of Bot., XIII., 1899. 



