102 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, hi 



CHAPTER XXX. REGULATION OF TRANSPIRATION IN 



LAND-PLANTS 



The regulation of the transpiration, and the checking of it at critical 

 times is, especially in xerophytes, effected by the following methods : 



1. Anatomical structure controlling transpiration. 



2. Diminution of the evaporating surface, either by movements, or 

 by reduction in the surface of leaves or of shoots which become irrever- 

 sibly and characteristically adapted to the prevaihng conditions. 



3. Regulation of illumination of the assimilating organs, either by 

 their assumption of a temporary profile-He (accomplished by photometric 

 movements dependent on intensity of illumination), or of a permanent 

 profile-lie (as in compass-plants). 



4. Investing organs, such as hairs, leaves, and the like, which weaken 

 the light as well as directly decrease transpiration. 



5. Ablation of rain-water from the leaves. 



I. ANATOMICAL STRUCTURE REGULATING TRANSPIRATION 



In this respect there is a fundamental distinction between land- 

 plants and water-plants. It is clear that a great difference must exist 

 between the surface of a plant that is permanently enveloped in water 

 or moist air, and one that is surrounded by dry air and engaged in intense 

 transpiration. But the difference concerns not only the construction 

 of the integument and the aerating system, including stomata and inter- 

 cellular spaces, but also the chlorenchyma. 



A. Cuticular Transpiration. 



Transpiration is either cuticular or stomatal. We shall first consider 

 cuticular transpiration, which takes place through the external cell-walls 

 of the plant, or in most of the higher plants through the epidermis. In 

 connexion with the regulation of transpiration there occur the following 

 devices : 



Cuticle is the first important regulator of transpiration ; its thickness 

 is adjusted in accordance with the need on the part of the plant to limit 

 transpiration ; yet other conditions seem to play a part, for Bergen ^ 

 found that the cuticle of young leaves is more impermeable to water 

 than is that of old leaves. The cuticle of hydrophytes is as a rule very 

 thin and permeable, but that of xerophytes is thicker and often com- 

 pletely impermeable. The outer walls of the epidermis may be strongly 

 thickened and cutinized, and in some cases may even include crystals 

 of calcic oxalate or silica. The leaves, owing to the nature of the epidermis, 

 are often leathery and glossy, and this is a frequent and striking feature 

 in tropical (sclerophyllous) trees, but is also met with in temperate climes 

 in leaves of evergreen plants, such as Ilex Aquifohum, some Coniferae, 

 and Vinca. The polished surface reflects a portion of the incident hght 

 from the leaves, and may thus be of use.^ Cuticle is often provided with 

 fine processes, especially when the external wall is convex. Vesque ^ 



' Bergen, 19046. ^ Wiesner, 18766. * Vesque, 1882. 



