IRRITABILITY 215 



revolved horizontally around its long axis on a clinostat, 

 thus exposing it uniformly on all sides to gravity and to 

 light, its leaves will assume no uniform and definite posi- 

 tion. However, when the plant is illuminated from one di- 

 rection only v/hile it is being revolved on the clinostat, the 

 leaves will take a very definite position, such that the blades 

 are at right angles to the incident rays. 



Light and gravity work very closely together in determin- 

 ing the direction, size, and even form into which organs 

 and organisms grow. When one infiuence is reduced or ex- 

 cluded, e.g. light, there is a change in the organism in its 

 relations not only to the influence excluded but also to 

 those that remain. The influence of the one force or the 

 other may predominate in one set of organs — as the roots 

 are affected mainly by gravity, the leaves mainlj^ by light — 

 or they may be nearly or quite balanced. Profoundly as 

 they affect the parts sensitive to them, these forces are 

 only factors in the complex of influences, mechanical, nutri- 

 ent, and other, which together determine the character of 

 the organism. No figures as to the relative effectiveness of 

 gravity and light acting upon the whole organism can be 

 given, yet Czapek* presents some figures of the same objects 

 under like conditions, indicating the minimum times of ex- 

 posure for reactions to gravity and light. 



Avena sativa, (etiolated) 



Sinapis alba, hypocotyl (etiolated) 



Beta vulgaris, " " 



Zea Mais, primary root 



Helianthus annuus, hypocotyl 



Phaseolus multifiorus, epicotyl 



Phycomyces nitens, sporangiophore 15 



The direction of locomotion in motile plants is infiuenced 

 more by light than by any other one infiuence except that 

 of food or of other stimulating compounds. An organism 



* Czapek, P. Weitere Beitrage zur Kenntniss der geotropischen Eeiz- 

 bewegungen. Jahrb. f. wise. Bot., Bd. 32, p. 185, 1898. 



