THE GENERAL CHEMISTRY AND PHYSICS OF PLANT- LIFE. 261 



Phenomena of growth and movement in the vegetable kingdom 

 are difficult to separate. Naturally there is movement with every 

 process of growth, though every movement is not accompanied by 

 growth. 



A. ACTIVE AND PASSIVE GROWTH. 



There is an active as well as a passive growth. The cambium- 

 ring of trees and the young portions of roots show the best examples 

 of active growth. The energy exerted by the growth-processes of 

 the cambium has not been definitely determined. From KRABBE'S 

 investigations 1 it would seem to be considerable in trees with decidu- 

 ous leaves ; the growth-pressure at certain periods rises to fifteen 

 atmospheres. 



The cortex also shows phenomena of passive growth induced 

 by the tangential tension proceeding from the cambium (KOPPEN),* 

 besides the active growth observed in the cork-cambium. A vis- 

 ible result of this tangential tension is the broadening of the 

 medullary rays in the cortex ; from this we may conclude that 

 mechanical tension can be converted into growth. It is easy of 

 demonstration that an originally straight stem may become perma- 

 nently crooked by processes of passive growth when the growing 

 portion of the stem is retained in a crooked position. Similar 

 processes occur in the winding of climbing plants and tendrils. 

 Such permanent curvatures are, however, induced by special ener- 

 gies which will be discussed later. 



B. THE EESULTS OF UNEQUAL GROWTH. 



According to DE TRIES (C. SCHIMPER), the term epinasty refers 

 to a relatively stronger growth of the upper side of an organ, hypo- 

 nasty to a stronger growth of the lower side. If these inequalities 

 occur in an organ growing in length, curvatures will appear; when 

 they occur in organs growing in thickness, as, for example, in 

 horizontal branches of trees, there is produced a woody body with 

 eccentric pith. 



Unequal growth of different tissues in one and the same organ 

 produces a series of phenomena which will now be briefly dis- 

 cussed. 



1 Abhandl. der Berl. Akademie, 1884. 



2 Nova Acta d. Ksrl. Leop. Akad., LIII. 



