54 A TEXTBOOK OF BOTANY [Ch. Ill, 7 



pear, is the primary function of the stem. It is true, not 

 all leaves thus attain full individual exposure to light, and 

 many are shaded more or less by others ; but within certain 

 limits this does not matter, for the reason, fully proved by 

 experiment, that a bright diffused light is quite as effective in 

 photosynthesis as direct sunlight, which contains in summer 

 more energy than leaves can utilize. 



Third of the adjustments is the presence of chlorophyll in 

 all practicable lighted parts. While leaves are preeminently 

 the chlorophyll-exposing organs, this function is by no means 

 restricted to them, but is shared in lesser degree by young 

 stems, young fruits, and even parts of the flower, though the 

 showy corolla and ripe fruits have other colors suited to their 

 special functions. It looks as though the plant took ad- 

 vantage of all its surfaces not needed in other functions to 

 spread to the light such chlorophyll as it can, even though 

 that be little. 



Fourth of the adjustments is the existence in plants of a 

 remarkable property of turning their green parts to the light, 

 no matter from what direction it comes. The fact is familiar 

 in house plants, which turn leaves and stems away from the 

 darker room towards the lighter window to a degree pro- 

 foundly affecting their forms, while the same power can be 

 proved in many striking ways by simple experiments (Fig. 25). 

 The younger parts of stems bend over until they point 

 towards the light, carrying with them the young leaves, which 

 independently set their blades at right angles to the light. 

 This bending is effected by growth, which becomes more active 

 on the side necessary to swing the stems to the light, and in 

 those parts of petioles necessary to swing the blades across 

 the light. Obviously the light does not effect the bending, 

 for that is accomplished by the plant through its own dif- 

 ferential growth; but the growth is made in response to 

 the greater intensity of the light, which therefore acts 

 as the stimulus to the bending. This process is called 

 phototropism (formerly heliotropism), and it is typical of 



