Surface Reduction and its Significance as a Hydroecological Factor

The plants examined are seasonally dimorphic. Their larger winter leaves fall off in the early summer and are replaced by smaller summer leaves. There is only a short transition period when both kinds of leaves are present on the plant. As the total water output of the plant is a function of the transpiration rate and the total transpiring surface, two questions arise: (a) do the summer leaves differ from the winter leaves in their transpiration rate and (b) does the replacement of winter leaves by summer leaves cause a marked change in the total transpiring surface? To find out whether the transpiration rate of summer leaves differs from that of winter leaves under the same environmental conditions, transpiration measurements were carried out in the transition period on both winter and summer leaves of the same plant on the same day. The measurements were made by the quick-weighing method (Huber, 1927) with a torsion balance and expressed as milligrams per gram fresh weight per hour. The daily averages of the transpiration rates of the two kinds of leaves of some plants are given in Table 1.