LEAVES 



535 



and the shape, size, and compactness of the cells may be modified con- 

 siderably as conditions change. 



Light and palisade development. — It was discovered long ago that 

 the gemmae of Marchantia develop chlorenchyma on whichever side is 

 exposed to the light, colorless tissue with rhizoids developing on the 

 , f other side ; also that the chlorenchyma develops 

 equally on both sides of a Thuja branch if ex- 

 I p posed equally to light, the palisades commonly 

 developed on the upper surface being due to 

 greater light exposure. These experiments, to- 

 gether with similar experiments on the beech 

 . jf and on Laciuca and Eucalyptus, as noted above, 

 have given rise to the prevalent view that light 

 determines the devel- 

 opment of palisade 

 cells. Furthermore, 

 light often is regarded 

 as the chief factor con- 

 cerned in the orienta- 

 tion of palisade cells. 

 Sometimes palisades 

 are oblique in vertical 

 organs, as in the leaves 

 of Typha and of Iris, 

 and in the stems of 

 Juncus and of Sali- 

 cornia (fig. 772), thus 

 giving rise to the view 

 that palisades tend to 

 become elongated in the direction of the incident light. However, most 

 vertical leaves and stems have transverse rather than oblique pali- 

 sades, growth under diverse conditions rarely having any marked 

 effect upon palisade orientation.' In those leaves of Isolepis that 

 hang vertically downward there are oblique palisades, but they point 

 down instead of up, indicating that their direction is related to leaf 

 structure and not to sunlight, and the same probably is true of most 

 oblique palisades. Furthermore, ordinary palisade cells are transverse 



' In a few plants the more exposed individuals have the more oblique palisades (as in 

 Saxifraga granulata). 



771 -5- 



Figs. 770, 771. — Cross sections of leaves of the prickly 

 lettuce {Lactuca scariola): 770, a section through a leaf 

 grown in the sunlight and thus exposed to strong trans- 

 piration ; both surfaces have been exposed directly to the 

 sun, and the chlorenchyma is composed entirely of palisade 

 cells, though those at the center of the leaf (p') are shorter 

 than are those near the periphery (p) ; the cuticle (c) also 

 is prominent; 771, a section through a leaf grown in deep 

 shade and thus not exposed to strong transpiration; the 

 chlorenchyma is composed entirely of sponge tissue (/), 

 and the cuticle (c) is but slightly developed; the shade 

 leaf also is much thinner than is the sun leaf; 0% stoma; 

 figures equally magnified. 



