122 AERATION AND AIR-CONTENT. 



xerophily originally produced by winter cold. Several investigators 

 have shown that Larix has a higher transpiration than many broad- 

 leaved deciduous trees, and it is practically certain that it is not a 

 xerophyte at all. 



Stopes (1907 : 48) has stated that the histological structure of 

 gymnosperms is incapable of allowing a rapid flow of water through 

 the wood, and hence the plants must set strict limits to leaf-surface 

 and transpiration. Even though growing with leafy deciduous trees 

 in a mesophytic community well supplied with water, the conduction 

 of the latter through the woody stem is insufficient for anything but 

 xerophytic foliage. Consequently, the xerophytic characters of coni- 

 fers are regarded in very many cases as not adaptations to xerophytic 

 conditions at present, or as inherited from the remote past as ves- 

 tigial characters, but as the result of physiological limitations of the 

 type of wood. A comparative study of transpiration in conifers and 

 deciduous trees in various habitats and altitudes is now under way 

 at the Alpine Laboratory, and it is hoped that this will throw new 

 light upon the nature and causes of xerophytism in conifers. 



Groom (1910 : 251) concluded that the xerophytism of conifers was 

 partly architectural in nature, as shown by the fact that the aggre- 

 gate leaf-surface of the conifer is often much greater than that of 

 the dicotyl tree, although the individual leaf is small. Despite the 

 low rate of transpiration for a single leaf or a unit of its surface, at 

 least some north-temperate conifers expend and need as much water 

 as do some dicotylous trees. The aggregate leaf-surface of cold- 

 temperate conifers is such that even their xeromorphic and xero- 

 phytic leaves do not prevent numbers of species from succumbing 

 from desiccation or growing feebly in places where ordinary dicotyl 

 trees thrive. Such conifers are architectural xerophytes in which 

 the extensive aggregate surface of the tree makes it necessary for 

 the individual leaves to be xeromorphic in form and xerophytic in 

 structure. This enables them to live in regions where there is a sea- 

 son of physiological drought in situations varying from dry dunes 

 to moist forests and from arctic and alpine situations to tropical 

 sites. The tracheidal nature of their wood is not a bar to progress 

 and the adoption of the deciduous habit, for in the larch a rapid 

 transpiration current flows through it and the leaves transpire rap- 

 idly. In spite of the author's conclusions, however, conifers seem 

 to be chiefly winter xerophytes, and the great increase of total sur- 

 face for adequate photosynthesis in summer. 



According to von Hohnel, the trees transpired as follows in grams 

 of water per unit of air-dry leaf- weight from April 1 to September 31, 

 1879: Larch, 1,150; linden, 1,030; beech, 860; birch, 845; elm, 755; 

 oak, 660; maple, 520; spruce, 210; Scotch pine, 105; Austrian pine, 

 100; fir, 75. In rate of movement in the stem, Groom found rates 

 of 204, 233, 240 cm. per hour in the larch in contrast to a maximum 



