THE LIVING CELL 35 
PHOTOTROPISM is the response of living things to light of any 
source. ‘The stems of higher plants tend to grow toward the light 
and are, therefore, positively phototropic, whereas the roots grow 
away from the source of light and are so negatively phototropic. 
Fic. 17.—Venus fly-traps, Dionea muscipula, growing in field near Wilmington, N. C. 
(Photograph by Steckbeck and Palmer.) 
Grorropism is the response of protoplasm to the stimulus 
of gravity. Roots of Pteridophytes and seed plants invariably 
grow downward toward the center of gravity and so are positively 
geotropic. The fruiting organs of the fungi and the main stems 
of higher plants tend to grow perpendicular to the earth’s 
