THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. 265 



the longitudinal axis of the organ ; (3) the cells of the entire organ 

 may tend to twist. All are the result of processes of growth. 



According to Krabbe and Schwendener, the second and third 

 causes producing torsions are active in those adaptive torsions which 

 bring dorsiventral leaves in a favorable position with regard to light, 

 and which cause zygomorphic flowers to assume a suitable position 

 for being visited by insects. The living plasm may be so influenced 

 by gravity or by light 1 that the growth of the cell-wall of the indi- 

 vidual cell may increase or decrease in a direction diagonal to the lon- 

 gitudinal axis. This gives the individual cells a tendency to become 

 twisted. According to these authors, there is therefore besides helio- 

 tropism a heliotortism, besides geotropisrn a geotortism. Under nor- 

 mal conditions gravity alone is active in causing plant-organs to assume 

 a definite position with regard to its supporting axis or the radius of 

 the earth ; but in order that plant-structures may assume favorable 

 positions in regard to light, light and gravity must act together, at 

 least in a number of instances, while in other instances light alone 

 is capable of bringing about the necessary torsion. I will add a few 

 more statements from the important work of SCHWENDENER and 

 KRABBE to show how readily superficial considerations seem to 

 make it possible to explain from a physical or mechanical basis the 

 most important phenomena of plant-life. 



1. An immediate causal relation between the adaptive move- 

 ments of leaves and flowers on the one hand and light and gravity 

 on the other does not exist ; rather the cause and effect are linked 

 together by the unknown irritability of plasm. 



2. The authors mentioned differ from DE VRIES in the explana- 

 tion of the phenomenon due to the amputation of leaves. Among 

 many of the plants with crossed opposite leaf-pairs (Philadelphus) 

 the leaf-blades on the horizontal branches are brought into the 

 required light-position by a torsion of the internode ; in addition 

 the leaves must also become twisted upon the petiolar axis to bring 

 the leaf -blade into the proper position. The active factor in bring- 

 ing about these movements is the sunlight. If the lower leaf of 

 the normal internode (not twisted) is removed, torsion will proceed 

 as usual ; but if the upper leaf is removed no torsion takes place in 



1 According to Schwendener and Krabbe, there are torsions due to the com- 

 bined influence of light and gravity. The manner in which these two forces act 

 is little understood. (Compare the subsequent statements in the text.) 



