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 while 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 influence 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 mainly 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. 



GEOTROP. HELIOTROP. 



Avena sativa, ( etiolated ) 15 minutes 7 minutes 



Sinapis alba, hypocotyl (etiolated) 15 " 10 " 



Beta vulgaris, " 15 " 10 " 



Zea Mais, primary root 20 " 



Helianthus annuus, hypocotyl 20 ". 20 " 



Phaseolus multiflorus, epicotyl 50 " 50 " 



Phy corny ces nitens, sporangiophore 15 " 7 



The direction of locomotion in motile plants is influenced 

 more by light than by any other one influence except that 

 of food or of other stimulating compounds. An organism 



* Czapek, F. Weitere Beitrage zur Kenntniss der geotropischen Reiz- 

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



