5*2 



ECOLOGY 



are of significance in relation to transpiration, since the closely appressed leaves of 

 desiccated individuals have a greatly reduced aggregate surface. 



Epinasty and hyponasty. If a leaf exhibits curvatures by reason of greater 

 growth at the upper surface, it is said to display epinasty, while if the greater 

 growth is at the lower surface, it displays hyponasty. Various plants, as Juniperus 

 (fig. 838) and Sempervivum, manifest hyponasty in the autumn, the erected leaves 

 becoming closely appressed to the stem or to one another, while epinasty the fol- 

 lowing spring results in leaf horizontality and increased surface exposure (fig. 



FIGS. 838, 839. Shoots of the juni- 

 per (Juniperus communis), showing vary- 

 ing leaf orientation : 838, a shoot as seen 

 in winter; note the ascending or erect 

 leaves, whose orientation probably is due 

 to a preponderance of growth beneath 

 (hyponasty) ; 839, a shoot as seen in sum- 

 mer; note the spreading leaves, whose 

 orientation probably is due to a pre- 

 ponderance of growth above (epinasty). 



FIGS. 840, 841. Rosettes of the 

 peppergrass (Lepidium), showing vary- 

 ing leaf orientation: 840, a winter ro- 

 sette with leaves closely appressed to 

 the ground through a preponderance of 

 epinastic growth ; 841, the same rosette, 

 after a stay of several days in a green- 

 house ; most of the leaves have an erect 

 or ascending orientation by reason of 

 a preponderance of hyponastic growth. 



839). In other cases, especially in rosette plants (as Lepidium), the reverse is 

 seen, the winter leaves being horizontal and closely appressed to the ground 

 as a result of epinasty (fig. 840), while spring hyponasty results in their 

 erection (fig. 841). In all cases the autumn reaction results in a reduced 

 surface exposure, and hence is favorable to protection from cold and from 

 excessive transpiration, while the spring reaction results in an increased sur- 

 face exposure, and hence facilitates synthesis. 



Leaf fall. The absciss layer. Leaves differ greatly as to duration, 

 most cotyledons and many xerophytic leaves living for only a few 

 weeks or even days, while evergreens may retain their leaves for a year 

 or two, or even for ten or more years, as in the pines and the cycads. 

 In most deciduous trees and shrubs the leaves remain for some months, 



