50 THE ECOLOGY OF LEAVES 



of all, perhaps, is the periodic reduction brought about by leaf fall 

 which will be discussed in paragraph 30. Many plants, however, 

 have a permanently reduced transpiration surface. The extreme 

 condition is represented by leafless plants in which the photo- 

 synthetic function is performed by stems. Among plants with leaves 

 there are all variations from water plants with leaves that are only 

 one or two cells thick to plants which have leaves that are freely 

 exposed and which are round in cross-section and so have a mini- 

 mum of surface in proportion to volume. Of course any reduction 

 in transpiration surface reduces also the photosynthetic surface 

 but the condition of each plant represents a compromise between two 

 major needs— one for free gaseous exchange for photosynthesis and 

 the other for protection from excessive loss of water. 



Some plants, especially members of the legume family, exhibit 

 interesting leaf movements commonly called sleep movements. 

 These consist of a closing up of the compound leaves during the 

 night or sometimes during the hottest part of the day in time of 

 drought. The leaves of the sensitive plant {Mimosa) not only close 

 during the night but they close quickly whenever anything comes in 

 contact with them. This movement is brought about by a mechan- 

 ism at the base of the petiole (and also at the base of each leaflet) 

 called the pulvinus. The decrease in turgidity of the cells in the 

 pulvinus as the water suddenly passes from them into the adjoining 

 intercellular spaces causes the leaves to close. They slowly resume 

 their former position when the water reenters the cells. In some 

 cases, as in bean leaves, the closing is due to an increase of turgidity 

 which is greater on one side of the pulvinus than on the other. No 

 satisfactory explanation of the causes of these increases and de- 

 creases of turgidity has yet been made. The closing of leaves during 

 drought reduces the exposed surface and probably aids in checking 

 water loss. The more common cases of night closing, however, and 

 the sudden movements of ]Mimosa leaves are probably of no real 

 benefit to the plants. 



30. Leaf Fall.— A deciduous tree is one that drops all of its 

 leaves at the beginning of a cold or a dry season. Contrasted with 

 these are the evergreen trees which drop their leaves only after a 

 new crop has been produced and so always have some green leaves. 

 In any case the fall of the leaf is brought about directly by the 

 formation of a special separation layer, called the absciss layer, at 

 the base of the petiole. The absciss layer consisting of two or more 



