Power to Adjust Parts to Surroundings 



235 



faces, as a rule, directly to the light like the leaves, as anyone 

 can observe in our house plants, or in those that happen to grow 

 close to a building (e. g. a border of Nasturtiums), or against 

 walls (e. g. Trumpet Creeper), or otherwise in one-sided light 

 (figure 80). In a few flowers (e. g. Sunflowers), the phototropism 

 even extends to the following of the sun through the day, though 

 the adjustment is only moderately effective. Perhaps at first 

 thought it will not be evident why flowers are phototropic at all, 



Fig. 79.— The adjustment of Ivy leaves (of English Ivy) into one plane, affording the best 

 aggregate exposure to light. (Copied, reduced, from Kernel's Pflanzenlehen.) 



because, unhke the leaves, there is nothing in the function of the 

 flower requiring the action of light. But on further contempla- 

 tion of the use of the flower (a subject to be fully explained in 

 the chapter upon Cross-pollination) , and especially of the function 

 of the showy corolla as an advertisement to show insects its 

 position, the matter becomes evident; because obviously this 

 function of conspicuousness requires that the corolla must stand 

 out where the light can strike on it most fully. As to fruits, they 

 are as a rule indifferent to light, though responsive to some 

 other kinds of stimuli, as will later appear. One special case, 

 however, deserves mention because illustrative of an additional 

 fact about stimuli. There grows in Europe a little cliff-dwelling 

 vine, Linaria Cymbalaria (figure 81), which turns its flowers as 

 usual to the sun, but its ripening seed-capsules away therefrom. 



