LEAVES 



543 



upward, so that the avoidance of shading is very striking. In many 

 plants, especially in those in which petioles are wanting, stem plasticity 

 may result similarly in advantageous leaf orientation; stem elongation 

 is comparable to petiole elongation, and stem twisting corresponds to 

 petiole curvature, resulting in favorable light relations for all leaves, 

 including those that originate on the under side of horizontal stems 



Figs. 780, 781. — ^ Branches of a honeysuckle (Lonicera); 780, an erect branch, 

 showing characteristic decussate phyllotaxy (p. 549); 781, a horizontal branch, in which 

 stem twisting has brought the leaves into a common plane, obscuring the decussate phyl- 

 lotaxy. 



(figs. 780, 781; also figs. 882-884, 895; see discussion of stems as 

 organs of leaf display, p. 645). 



Leaf mosaics. — Since each leaf assumes the best lighted position 

 possible, there is in general an absence of overlapping in the leafage 

 as a whole, and if the foliage is dense, most of the available leaf space 

 becomes occupied. The combined result of the absence of overlap and 

 of the maximum occupation of space often is spoken of as a leaf mosaic. 

 The perfection of this mosaic sometimes is enhanced by the fitting of 

 projecting angles into reentrants (as in Hedera), by the reciprocal ar- 

 rangement of unsymmetrical leaves (as in Celtis and in Begonia, fig. 895), 

 or by the intercalation of small leaves between larger leaves (as in Sela- 

 ginella, fig. 896). Excellent mosaics are seen in climbing plants (as in 



