534 ECOLOGY 



to environmental control. However, there are many cases of rigidity, 

 as in most monocotyls and in some dicotyls (e.g. in the Crassulaceae), 

 where no condition s'eems to induce palisade development; in various 

 dicotyls palisades appear to develop without regard to external factors. 

 In many cases palisades appear in the bud, where the usual palisade- 

 producing factors can have no influence. But even in these cases there 

 often is variation in cell length, in the number of palisade layers, or 

 in compactness of tissue. 



Among the most plastic forms are the amphibious plants, such as 

 Proserpinaca and various buttercups, in which the submersed leaves 

 have no palisades, the entire mesophyll consisting of loose sponge, and 



768 769 v 



FIGS. 768, 769. Leaf cross sections of the mermaid-weed (Proserpinaca paluslris") : 

 768, a section through a mesophytic air leaf, showing prominent palisades (p) and epi- 

 dermis (e), and a stoma (s) ; 769, a section of a segment of a water leaf, showing undif- 

 ferentiated chlorenchyma (c) made up of sponge cells, between which are prominent air 

 spaces (a); the epidermis and the conductive tract (v, not shown in 768) are much re- 

 duced in the water form; figures equally magnified. 



the epidermis often containing chlorophyll (fig. 769) ; the emersed leaves 

 are without epidermal chlorophyll and have well-differentiated sponge 

 and palisade regions (fig. 768). When Lactuca scariola is grown in 

 intense light, the leaves assume a vertical position and have palisade 

 layers on both sides (fig. 770); in diffuse light the leaves are horizontal 

 and have palisades only on the upper side, and in dense shade there are 

 no palisades at all (fig. 771). Equally extreme variation is seen in Euca- 

 lyptus globulus, in which the form of the leaf, as well as its position, 

 changes from shade to sunlight, the sun form having palisade layers both 

 above and below, while the chlorenchyma of the extreme shade form 

 consists entirely of sponge. Few leaves show such extreme variation 

 as is found in Lactuca, Eucalyptus, and in amphibious plants, but there 

 are many plants in which the upper leaf portion may have either pali- 

 sade or sponge, and many more in which the number of palisade layers, 



