24 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



Relation of Surface to Volume 



Huber (105) attaches great importance to the ratio between the 

 surface of the leaf (as measured in square centimeters) and the volume 

 (in cubic centimeters, as measured by multiplying the thickness of the 

 leaf by the surface). Since the number of cells transpiring into the 

 internal air spaces of the leaf will vary with the volume, while the 

 number of stomata varies with the surface, it is easy to see that the 

 relation between the surface and the volume is important, especially 

 in succulent leaves. Huber found that the figure 200 represents about 

 the maximum size of this ratio and also that, from an ecological point 

 of view, trees growing in drier sites tend to a smaller development of 

 the transpiring surface, although this effect of humidity may be 

 masked by the effects of light or other site factors. Thus Fagus 

 sylvatica leaves (shade) show a ratio of 153, Quercus pedunculata 

 Ehrh. leaves (sun) 84, and Q. ilex 70; the thickness of these leaves is 

 0.13, 0.21, and 0.28 mm, respectively. Conifers show an exceptionally 

 small ratio (30 to 50). 



Specific Variations 



As has been indicated above, there are great specific variations in 

 the transpiration of leaves, dependent in part, of course, on the factors 

 already mentioned. Thus, Haas and Halma (78) found a consider- 

 able variation in the transpiration of various Citrus species. Both 

 rooted cuttings and detached leaves of Eureka lemons transpired 

 more than Marsh grapefruit, which in turn transpired more than 

 Valencia oranges. The relative rates of water loss per unit surface 

 during August and September were about 4.5:3.25:3. Such figures, 

 however, have little value so far as specific differences are concerned 

 unless we know all of the other factors involved; but they are valuable 

 from the point of view of relative water loss and the resultant water 

 requirement, in connection with which subject they will be discussed 

 further. In fact the term "specific variations" is, for the most part, 

 a subterfuge to hide ignorance as to the real causes. 



CONIFERS AND BROAD-LEAVED TREES 



Broadly speaking, as shown by some of the previous workers 

 mentioned, there is a distinct difference between the transpiration 

 rates of conifers and broad-leaved trees. During the active growing- 

 season the broad-leaved trees transpire much more rapidly than do 

 the conifers, but the latter transpire over a much longer period of 

 time; so that in considering total water utilization one must take 

 into account not only the rate of transpiration but also the duration. 



Weaver and Mogensen (229) studied 2- to 4-year-old seedlings (in 

 pots) of Pinus banksiana, P. ponderosa, P. murrayana Balf., Abies 

 grandis Lind., Picea engelmanni, Pseudotsuga taxifolia, Acer sacchari- 

 num L., Quercus macrocarpa, and Ulmus americana L. In the middle 

 of the summer the broad-leaved trees transpired per unit area about 

 2.8 times as much as the conifers, but in the fall the conifers lost 

 about as much per unit area as the broad-leaved trees or even more. 

 In the winter the conifers transpired only 0.5 to 2 percent as fast as 

 in the autumn, even with sufficient soil moisture — a transpiration 

 rate only slightly greater than that of defoliated deciduous trees. 



The difference between the transpiration of conifers and hard- 

 woods has been observed for many years. One of the earliest workers 

 in this field was Lawes (133), who, at intervals over an entire year, 



