LEAF ADJUSTMENT 



49 



response to the influence of light is called heliotropism, 

 a word that means " turning to or with the sun." 



58. Leaf Drainage. — Another important adjustment 

 that leaves undergo is in regard to water. Notice the 

 leaves of tulips, hyacinths, beets, 

 turnips, and of bulbs and plants 

 generally whose roots do not spread 

 in a horizontal direction, and it 

 will be found that their leaves usu- 

 ally assume a position more or less 

 like that shown in Figure 93. 

 Their edges are apt to curve in- 

 wards and they slope from base 

 to apex at such an angle as to 

 carry most of the water that falls 

 upon them straight to the axis of 

 growth, and so on down to the 

 root. In most trees and shrubs, 

 on the other hand, and in plants generally with spreading 

 roots, the leaves slope from base to tip so that the water 

 is carried away from the axis to the circumference, where 

 the delicate young root fibers grow that are most active 

 in the work of absorption. In the first case the drain- 

 age is said to be centripetal, or towards the center of 

 growth ; in the second, it is centrifugal, or away from the 

 center. 



95- — Leaf with tapering 

 point that acts as a gutter in 

 conducting off water. 



59. Leaf Cups. — The water could not well run down a 

 long, slender leaf stalk from the blade to the stem, hence, 

 in plants fitted for centripetal drainage the leaves are gen- 

 erally sessile, or the petioles are grooved or appendaged in 

 various ways, as in the winged leaf stalks of the sweet pea 

 and the common leaf cup (^Polymnia), which takes its 

 name from the cuplike expansion into which the base of 

 the petiole is often dilated. Connate leaves may also 

 serve the same purpose. Can you think of any other 

 probable use for these natural water holders ? Why, for 



ANDREWS'S EOT. — 4 



