CHAP. XXX TRANSPIRATION IN LAND-PLANTS 113 



increasingly acute ; they are thus relatively less illuminated and heated, 

 and transpiration is inevitably decreased. These movements are executed 

 by the compound leaves of numerous plants, and particularly of some 

 growing in tropical, dry bushlands ; among such are many species of 

 Acacia, and other Mimoseae, Papilionaceae, Oxalidaceae (including Oxalis 

 Acetosella), Zygophyllaceae ; but simple leaves of some plants, including 

 Hura crepitans, hkewise execute movements dependent on the intensity 

 of hght.^ The leaves of the plants in question are wont to be more or 

 less mesophytic in structure, for example, the leaflets of species of Acacia 

 in the West Indies ; acacias endowed with the power of movement 

 in response to intensity of hght are often or always thin, and have 

 a smooth, thin epidermis.- 



Temporary vertical positions that must be of utility to the organs 

 concerned are met with in connexion with many or most young developing 

 leaves ; for as they shoot from the bud they are vertical, and sometimes 

 remarkably so. 



B. Fixed Lie in Relation to Light. 



All foliage leaves as they unfold execute movements as a result 

 of which they assume a favourable fixed he, in regard to which Wiesner 

 has conducted investigations for many years.^ They place their blades 

 perpendicular to the strongest diffuse light. In unusual circumstances, 

 when more intense light prevails, they assume a profile-lie. A diminution 

 in the action of the sunhght, and consequently in transpiration, will 

 result from a permanent profile-pose or other similar arrangements 

 on the part of assimilating surfaces, which thus receive the intense midday 

 light at acute angles. This is the case with the so-called ' compass-plants ', 

 represented in northern Europe by Lactuca Scariola, the leaves of which, 

 in places exposed to strong sunlight, place themselves in the meridian 

 with their faces vertical^ ; as a North American ' compass-plant ', Silphium 

 laciniatum may be mentioned. 



Leaves exposing their edges to the light are met with in many other 

 plants, for instance, in some species of Eucalyptus, phyllodinous species 

 of Acacia, and Proteaceae, in Austraha ; species of Statice in South 

 Africa ; Laguncularia racemosa in the West Indies ; and Bupleurum 

 verticale in Spain. 



Leaf-blades that are erect, or directed sharply upwards, are common 

 among xerophytes growing in intense sunlight : for instance, in the 

 West Indian Coccoloba uvifera,^ in many grasses (including Brachypo- 

 dium ramosum, Festuca ovina), in Calluna, Peucedanum Cervaria,^ and 

 Hehchrysum arenarium ; among marsh- and moor-plants may be named 

 Iris Pseudacorus, Narthecium ossifragum, and Totieldia ; among halo- 

 phytes, Rhizophora and other mangrove plants.' More rarely, blades 

 hanging vertically downwards are met with. The switch-plants may 

 again be mentioned as being designed upon a very similar plan. 



Corrugations and folds in the lamina may play the same part, and 

 become more frequent the drier the chmate is : as examples may be 



' See C. Darwin. 1880. '' Warming, 18996. ' Wiesner, 1876. 



* Stahl, 1880-81. " Illustrations in Borgesen and O. Paulsen, 19CXD. 



" According to Altenkirch, 1894. ' Illustrations in Joh. Schmidt, 1903 



WARMING I 



