214 PLANT PHYSIOLOGY 



Most stems are positively heliotropic, that is, bend toward 

 light of ordinary intensity and composition. The English 

 Ivy ( Hedera ) , to a certain extent Ampelopsis, the touch- 

 me-not (Impatiens), the garden nasturtium in its older 

 stages ( Tropceolum majus ) , * and some other plants, are 

 negatively heliotropic toward intense light, though not to 

 light of lower intensity. These are plants which either 

 climb against objects which absorb much light and hence 

 appear dark, or thrive best in comparative shade. The 

 behavior of the roots which attach these climbers to their 

 supports is consistent with the relation of most roots to 

 light. 



Roots are not markedly sensitive to light, light serving, 

 as a rule, only to supplement gravity. So niuch more 

 does gravity influence the direction of growth of roots that 

 the influence of light is scarcely apparent t until all parts 

 are uniformly subjected to gravitation by means of the 

 clinostat. 



Underground and other horizontally growing stems exhibit 

 a peculiar adjustment to the influences of gravity and light, 

 their position being the resultant of their reactions to these 

 two opposite stimuli. The prostrate shoots of Rubus, J the 

 runners of strawberry, etc., and the root-stocks of Nuphar^ 

 are examples. Goebel shows that the root-stock of Nuphar 

 luteum creeps horizontally in the mud, possessing at this 

 time a somewhat dorsi-ventral structure. If covered with 

 soil, such a rhizom would bend, grow vertically upward 

 until it came to the light, the structure of the vertical part 

 being radial. At the surface of the soil it would bend to 

 the horizontal and grow through the muddy twilight as 

 before. 



The position and direction of growth of leaves is also a 

 resultant of the two forces, gravity and light. If a plant is 



* Wiesner, J. Die heliotropischen Erscheinungen. Denkschrift der Wiener 

 Akademie, Bd. .39 u. 43, 1878 u. 1880. 



\ Wiesner, J. /. c. 



X Czapek, Fr. ;. c. 1898, p. 245, 257, etc. 



§ Goebel, K. Organographie der Pflanzen, I., p. 198, 1898. Stahl, E. 

 Einfluss des Lichtes auf den Geotropismus einiger Pflanzenorgane. Ber. d. 

 Deutsch. Bot. Geeellsch.. Bd. II., 1884. 



