160 On the Transpiration of Plants. [ March, 
great, indeed, in no essential are they different, except that 
the former usually goes forward in a bilaterally symmetrical way, 
while in the latter it most frequently does not. 
The preceding facts and considerations embrace what may be 
regarded as the complementary principle demonstrating the 
mechanical theory of axial segmentation or origin of vertebrz, 
as proposed by Spencer, since it must be allowed, that if segmen- 
tation is due to flexures of the vertebral axis, conversely, union, 
coossification of segments, is due to their absence, because oppo- 
site conditioning causes must produce opposite effects in two 
things respectively so conditioned. 
:0: 
ON THE TRANSPIRATION OF PLANTS! 
BY J. M. ANDERS, M.D., PH.D. 
N looking over the literature of the subject, one is surprised to 
find how little definite knowledge we possess in regard to 
the process of plant transpiration. When the importance of the 
subject is considered, there would seem to be no explanation for 
this apparent omission of research. 
It has been pretty well established that transpiration is pro- 
duced and modified by influences acting from without, and by the 
structural peculiarities of the plant. Most important among the 
- former modifying agencies are sunlight, wind, dew point and 
temperature ; and among the latter circumstances is to be men- 
tioned, more particularly, the nature of the epidermal tissue. The 
precise connection between these various conditions and the 
amount of water evaporated has not been investigated to any con- — 
siderable extent; and the most important question, viz.: the 
amount of liquid ordinarily transpired by different plants has, 
also, hitherto been quite as sadly neglected. 
A few bare statements are made in relation to the quantity of 
transpiration (Gray’s Structural and Systematic Botany). A sun- 
flower 314 feet high, with a leaf surface of 5616 square inches, when 
exposed to the air, evaporated from 20 to 30 ounces in twelve 
hours, being seventeen times as fast as man exhales. A seedling 
apple tree, with leaf surface amounting to 112 square feet, evapo- 
1 The Geo. B. Wood prize essay, 1877, read before the Society of the Alumni of 
> the Auxiliary Dept of Medicine, University of Pennsylvania. 
