LEAVES 



549 



chlorenchyma occurs on whichever side is the more exposed to light. 

 Surface expansion is almost universally characteristic of leaves, and 

 though perhaps not caused by light, it is none the less of prime im- 

 portance in light reception. 



Vertical leaves. In most of the leaf forms above considered there is 

 a tendency toward the display of a maximum surface to incident light, 

 In vertical leaves and also in leafless stems the exact reverse occurs. 

 In the latter the chief limiting factor is not insufficient light, but rather 

 too great transpiration ; perhaps, too, the excessive light may be directly 

 injurious to the chlorophyll and hence to synthesis. The peculiar 

 reaction of the compass plant has been supposed to be particularly 

 advantageous, since a leaf facing east or west misses the intense zenith 

 rays and yet has the full benefit of the weaker rays of morning and 

 evening. The situation in Nicotiana and Verbascum is still more com- 

 plicated, there being from the base to the apex a series of leaves varying 

 in form and position in apparent 



correspondence with the increasing Yj/ 



light intensity; the basal leaves seem - *A 



fitted for maximum light reception, 

 and the apical leaves for maximum 

 light avoidance. ,. 



The position of leaves on stems. 



Light reception is facilitated by various 

 features of leaf structure and arrange- 

 ment, which have little or no causal 

 connection with light. While petioled 

 leaves usually have considerable plas- 

 ticity in their orientation and thus are 

 relatively free from disadvantages due to 

 place of origin, the particular orientation 

 of sessile leaves often is determined by 

 their stem position or phyllotaxy. Leaves 

 usually are arranged in cycles (whorls) or 

 in spirals. A simple and common ar- 

 rangement, known as decussate, is that 

 in which two-leaved cycles alternate with 

 one another, resulting in four vertical 

 rows (arthostichies) of leaves (fig. 780). 

 There are many systems of spirals, the 

 simplest being the distichous or \ ar- 

 rangement (i.e. there are two orthosti- 



FIGS. 787, 788. Experimental modi- 

 fication of the phyllotaxy in the luminous 

 cave moss (SchistostegaOsmundacea): 787, 

 a shoot which, after the development of 

 ordinary distichous leaves (/), has been ex- 

 posed to feeble illumination; the new leaf 

 arrangement (r) is spiral; 788, a shoot 

 which from the first has been exposed to 

 feeble illumination, hence exhibiting spiral 

 arrangement throughout. From GOEBEL. 



