WATER UTILIZATION BY TREES 63 



endemic species always fall in dry periods. This question was also 

 studied by Kamerling (118) who cut off leafy twigs from various 

 species in Java and compared the transpiration of those leaves which 

 ordinarily are shed during the dry periods with those which are not. 

 On the average the trees which shed their leaves during droughts 

 showed a much stronger transpiration and were less able to regulate 

 their transpiration than the trees which retained their leaves in dry 

 periods. 



Increase in volume at the expense of surface or the development of 

 succulence is of very slight importance in trees, but it has been noted 

 that the mesophyll is commonly more compact in trees in arid regions. 

 In Eucalyptus, for instance, the leaf tissues of the species found along 

 river banks and in humid valleys generally have a much larger propor- 

 tion of intercellular space than those of species which grow in drier 

 regions. These differences in structure are obviously related to the 

 sun and shade differences previously mentioned. 



A change of position in leaves in the light is also seen in Eucalyptus 

 species, many of which place the leaves in a vertical position when 

 exposed to light, as opposed to the horizontal position of normal leaves. 

 Certain species of Acacia behave similarly, and the forests of Australia, 

 where these genera are common, are sometimes spoken of as "shade- 

 less". The vertical leaves of Eucalyptus globulus, however, have a 

 structure somewhat different from that of the horizontal ones. The 

 former have a thick cuticle, while the latter have a more waxy coat- 

 ing, according to Johnson (116), who also reports that, contrary to 

 common opinion, the vertical leaves transpire more than do the hori- 

 zontal ones. Since most workers have held a different view, this 

 problem apparently deserves further study. 



Trees that come more within the scope of this publication are the 

 juniper and linden. Stahl (202) calls attention to the fact that in 

 Juniperus virginiana L., in sunny sites the scales are shorter and. 

 more closely appressed, while in shady habitats or in the interior of 

 the tree, the scales are more leaf like and more loosely grouped 

 together. A similar condition is cited by Kerner (121), who states 

 that the leaves of Tilia alba Michx. and T. tomentosa Moench have an 

 almost vertical position on branches and twigs exposed to the sun ; but 

 in shaded regions, the leaves remain much more nearly horizontal. 

 The curling up of the edges of leaves at the beginning of wilting is a 

 very common occurrence and probably protects the tissues somewhat 

 against further drying out. 



Another common protection against excessive transpiration is the 

 thickening of the cuticle and of the outer epidermal w T alls, resulting 

 frequently in leaves that are tough or leatherlike. This is often seen 

 in broad-leaved evergreens and is probably a protection against the 

 physiological dryness of the winter season. This thickening of the 

 walls of the epidermis sometimes proceeds so far that all that remains 

 of the cell lumen is merely a point, as in Scotch pine, or only a thin 

 line, as in the mountain pine. 



The cuticle also may be covered with wax, forming a shiny, smooth 

 layer as in Quercus sessiliflora, or with a dull covering as in Q. pedunc- 

 ulata. This type of protection is quite common also in broad-leaved 

 evergreens as well as in conifers. The glauca varieties of conifers are 

 so called from the bluish-white waxy bloom on the leaves. Many 

 trees, such as Salix jragilis L., Amygdalus communis L., and Acer 



