582 



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 surfa.ce, it is said to display epinasty while, if the greater 

 growth is at the lower'surface, it displays hyponastv. ^Various plants, as Juniperus 

 (fig. 838) and Sempervj/vum, 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). 



!' 841 



FIGS. 840, 841. Roset*es 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 man xerophytic leaves living for only a 



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, 



