LEAVES 539 



periments show that the average synthetic efficiency of palisades in pro- 

 portion to sponge varies from a ratio of one hundred to thirty-six in 

 xerophytic leaves to a ratio of one hundred to ninety-two in relatively 

 homogeneous leaves like those of the bamboo. 



Palisade cells commonly are thought to be of advantage in connection 

 with carbohydrate synthesis, partly because light reception is favored by 

 the absence of cross-partitions and by the parallelism of the cells to inci- 

 dent light, and partly because cell elongation makes possible the motility 

 of a relatively large number of plastids, permitting their peripheral posi- 

 tion in diffuse light and their movement to some depth in intense light. 

 For the most part, however, palisade orientation has been seen to be 

 unrelated to light direction; furthermore, light commonly is present in 

 superabundance, especially in the very plants that have the most prom- 

 inent palisades, and it is in these same plants that plastid motility is 

 slightest. Another theory concerning palisade cells is that they reduce 

 transpiration because of their compact arrangement and the consequent 

 reduction of their lacunae, and also because of the increased leaf thick- 

 ness in proportion to the transpiring surface entailed by their presence. 

 This theory seems quite as plausible as the one first mentioned, but 

 the quantitative significance of the palisades in transpiration reduction 

 is uncertain. A third theory relates palisade cells to the conductive 

 bundles, their elongation being supposed to favor the rapid migration 

 of the products of synthesis; there is no adequate support for this theory. 

 A canvass of the situation fails to revealjainy conspicuous advantage 

 inpdisajde_cells. If their shape is due to crowding, as suggested pre- 

 viously, no especial advantage is to be expected. In any event, no 

 theory yet suggested has enough evidence in its favor to give it particu- 

 lar standing. 



3. THE RELATION OF LEAVES TO LIGHT 



Horizontal leaves. Growing leaves tend to present their surfaces, 

 especially their upper surfaces, to the incident light (fig. 776); hence, 

 leaves are called transversely phototropic or diaphototropic organs. The 

 simplest light relation is that in which the chlorophyll-bearing organ 

 is more or less parallel to a horizontal substratum, as in liverworts (fig. 

 742), in foliose lichens (fig. mi), in rosette plants without petioles 

 (fig. 1036), in floating plants (fig. 727), and in plants with floating 

 leaves (fig. 777). Even in such plants the shifting of the sun's position 



