LEAVES 537 



even indirectly. For example, palisades are much better developed 

 in dry than in wet soil, exposure to light being equal. Again, in 

 amphibious plants there usually is an abrupt change from air leaves 

 with strong pali ades to water leaves with no palisades, although the 

 change in light is gradual. 1 Furthermore, it has been shown experi- 

 mentally that palisades develop somewhat better in dry air with weak 

 light than in moist air with intense light. Apparently palisade develop- 

 ment increases with the transpiration, or, more precisely, when there 

 is an increase of transpiration in proportion to absorption. Even where 

 palisade development has been referred to light, it is possible that 

 transpiration is a factor, since increased light commonly is accompanied 

 by increased transpiration. The maximum development of palisades 

 occurs in deserts and in other dry exposed situations where high light 

 intensity is coupled with strong transpiration and relatively low absorp- 

 tion. Strong palisade development is seen also in alpine habitats, where 

 transpiration is relatively high because of the low absorption in the cold 

 soil. 



The transpiration theory accounts satisfactorily for the extreme development of 

 palisades in plants of bogs and salt marshes. Salt marsh plants grow where root of 

 formation and absorption are difficult because of the concentration of the soil water, 

 hence their transpiration is high in proportion to their absorption. The case of 

 bog plants is more difficult to understand, though their poor root development indi- 

 cates hard conditions and suggests the likelihood of low absorption in proportion to 

 transpiration. The probable factors that tend to reduce absorption in bogs, either 

 directly or by reducing root development, are low soil temperatures, imperfect aera- 

 tion (especially oxygenition), and the toxicity of the bog waters ; it has been shown 

 that each of these conditions independently is capable of inducing palisades and 

 other xerophytic features in various species (as Rumex Acetosella) . It has been 

 suggested that the xerophytic peculiarities of bog plants may have been developed 

 elsewhere, but the fact that plants from without, when they are grown in bog con- 

 ditions, develop weak roots and small t ick leaves with prominent palisades (a com- 

 bination not characteristic of ordinary xerophytes) makes it probable that most of 

 the bog plants owe their xerophytic peculiarities as well as their sparse root system 

 to the bog itself, though it is not impossible that some xerophytes may have immi- 

 grated from other habitats. 



The precise action of light or of transpiration upon palisade development is not 

 known. In particular, the mechanical reason for cell elongation is obscure, but a little 

 speculation may be hazarded. Palisades have a more concentrated cell sap and hence 

 a greater turgor than have sponge cells. Usually high concentration results in cell 

 sphericity (see discussion of cell form inStigeoclonium, p. 591), but palisades depart 



1 In the bulrush, however, there is a gradual change, palisades occurring even beneath 

 the water surface ; light seems to be the chief factor here. . . 



