STEMS AND BRANCHES AND THEIR USES 193 



212. Influence of Light. — One stimulus that affects the 

 direction of growth of the parts of a plant is light. The stem 

 of an erect plant responds to the stimulus of light by growing 

 toward the point from which the light comes. If house plants 

 stand near a window, their stems grow toward the window. 

 Plants growing out of doors, imless they are close to a building 

 or other large object, receive light from above and from all 

 sides, but not from below ; and as a result of the many-sided 

 illumination they grow straight or nearly straight upward. 

 But even an out-of-door plant does not at any particular mo- 

 ment receive the same amount of light from all sides. On 

 a bright day, especially, the strongest light comes from the 

 east in the forenoon, from near the zenith at noon, and from 

 the west in the afternoon. If the growing parts of the stem 

 react quickly to the stimulus of light, the position of the 

 younger — that is, of the upper — parts of the plant will vary 

 from hour to hour. The sunflower, whose head of flowers 

 follows the sun in its daily course across the sky, shows 

 plainly the tendency to grow in the direction of the brightest 

 illumination. The older parts of the sunflower stem, which 

 are no longer growing in length, are erect because in the long 

 run they have received about as much light from the east 

 as from the west, and about as much from the north as from 

 the south. 



The far-reaching effect of light conditions upon the 

 growth of erect stems is seen in comparing the long, slender 

 trunks of trees growing close together in a forest where they 

 have been largely shaded by neighboring trees, with the 

 shorter, thicker, much-branched trunk of a tree of the same 

 species growing in an open field where it has been well 

 Hghted from all sides (see Figs. 64 and 65). The form 

 of the forest tree has also been influenced by the early 

 death of its lower branches whose leaves received too little 

 light to enable them to make the food that the branches 

 needed. 



