THE ROOT. 



73 



medium in whatever direction it may be advisable for 

 them to do so. Baillon observed the roots of Monster a 

 deliciosa growing vertically upwards away from the 

 heated atmosphere caused by the warm stage on which 

 he had placed the pot. 



The manner in which some modern physiologists 

 write about gravity is distinctly misleading ; one gets 

 the impression from their treatises that they regard it 

 as a primary stimulus comparable, for example, to that 

 of light, and one is led to suppo-se that roots grow 

 downward under the influence of gravity in the same 

 way as stems, under the influence of light, grow toward 

 the source of light. Nothing, however, can be further 

 from the truth; gravity, as a primary cause, has nothing 

 whatever to do with a root's movements. Light, 

 however, is a primary cause of heliotropic movement 

 because the plant receives a direct benefit from the 

 rays striking its leaf-surface at a certain angle. 



The true cause of root-movements resides in the 

 habit and needs of the plant, built up upon the past 

 experience of the race ; and according to its needs, of 

 fixture, moisture, food, aeration, temperature, water- 

 currents, etc., will it cause its roots to grow in this or 

 that direction : vertically upwards just as readily as 

 vertically downwards. Gravity only comes into play 

 in connection with the minute mechanism by which the 

 main movement is accomplished ; it acts on the cell- 

 contents, thereby stimulating the cells and tissues.* 



4. ADVENTITIOUS ROOTS. 



Included under this heading are all roots which 

 occur out of their normal position, or at a period, or 

 under circumstances, exceptional for them. Adven- 

 titious roots may occur (1) on roots, (2) on stems, 

 (3) on leaves, (4) on floral organs, (5) on prothalli. 



* As a matter of fact scientific study is not concerned with the true 

 primary cause at all, but solely with the proximate mechanical one. The 

 teleological aspect of the subject belongs to the domain, not of science, but 

 of philosophy. 



