632 



ECOLOGY 



other xerophytes (as Ledum or Campanula, figs. 867, 868) is due to in- 

 creased cutinization and to palisade development. Apparently these 

 different xerophytic reactions are due to a common cause, namely, expo- 

 sure to increased transpiration. 



Not only does the reference of succulence to transpiration bring it into 

 line with other xerophytic characters, such as cutinization and palisade 



development, but like these, succulence 

 develops in analogous situations, as in 

 maritime habitats. Years ago it was 

 shown that Salicornia, one of the most 

 succulent of halophytes, loses much of 

 its succulence when grown where the 

 soil is poor in sodium chlorid, while its 

 succulence increases with the addition 

 of this salt to the soil. The same is 

 true of Glaux mariiima, Rhizophora, 

 and various other salt plants. The 

 dorsiventral leaves of the beach pea and 

 the wallflower tend toward equilaterality 

 in salty soils, precisely as they do when 

 exposed to strong transpiration. Simi- 

 larly, plants of alkali deserts (as Hali- 

 modendron) show a reduced development 

 of water tissue when grown in ordinary 

 garden soil. In so far as succulence 

 involves increased cell lurgidity and 

 sphericity, the influence of transpiration 

 or of salt solutions of high osmotic pres- 

 sure is essentially the same as in Stigeo- 

 clonium, where these characteristics ac- 

 company high concentration of the cell 

 sap, however produced (p. 591). Usually a desert plant is exposed to high 

 transpiration, and often absorption is slight; such conditions result in cell- 

 sap concentration, which favors succulence. Similarly, a Salicornia plant 

 in a salt marsh is exposed to transpiration, while at the same time the soil 

 salts make absorption difficult, also resulting in cell-sap concentration. 



The problem of water accumulation seems to present greater difficulties than 

 does that of cell shape or turgidity, since it is difficult to see how increased transpi- 

 ration can stimulate water accumulation. However, the problem may have to do 



Fig. 928. — A cross section 

 through a Peperomia leaf, illustrat- 

 ing peripheral or epidermal water 

 tissue (w), the epidermis being three 

 or four layers thick and containing 

 mucilage glands (g) ; note that the 

 uppermost chlorenchyma layers (c) 

 consist of small closely packed cells 

 with abundant chlorophyll, while 

 the lowermost chlorenchyma layers 

 consist of larger and more loosely 

 placed cells with less chlorophyh; 

 highly magnified. 



