460 Prof. J. Larmor. On the [June 29, 
The value of 0°0077 gramme that we have adopted for assimilation in the open air when 
the CO,-pressure is the Jimiting factor, was obtained by Brown and Morris by this method, 
and is therefore subject to suspicion and is probably too high. 
The new data for arriving at this most important value, brought forward by Brown and 
Escombe, and based on measuring the intake of CO, by a leaf in a glass chamber in a rapid 
current of ordinary air, show a very wide range of variation even for the leaves of a single 
species (0°0016 to 0:0047 gramme per 50 sq. cm. for Polygonum Weyrichii). Primarily, 
the degree to which the stomata are open and, secondarily, the magnitude of the concurrent 
respiratory production of CO,, would seem to be the important factors in disturbing the 
inflow of CO, from without. 
The highest value thus arrived at is 0°0047 for a detached leaf, and as attached leaves 
with stomata less widely open give smaller values, the number adopted by us may be much 
too high. This would increase the waste. of photosynthetic radiation in Nature, but the 
information at present available is not sufficient to allow us to readjust the table 
on p. 455]. 7 
Note on the Mechames of the Ascent of Sap in Trees. 
By Professor J. Larmor, Sec. R.S. 
(Received June 29, 1905.) 
The following remarks, relating to one of the most powerful and universal 
of the mechanical operations of organic nature, are based mainly on the 
numerous experimental results reported in Dr. A. J. Ewart’s recent 
memoir.* Their chief object is to assert the view that we are not com- 
pelled to suppose the sap, in the column of vessels through which it rises, to 
be subject to the great actual pressure, amounting in high trees to many 
atmospheres, that is sometimes postulated. It is hardly necessary to remark 
that the problem of the rise of sap is one of mechanics, in so far as concerns 
the mode of the flow and the propelling power. 
Contrary to the view above referred to, it seems not unreasonable to con- 
sider that the weight of the sap in each vessel is sustained in the main by 
the walls and base of that vessel, instead of being transmitted through its 
osmotically porous base to the vessels beneath it, and thus accumulated as 
hydrostatic pressure. 
We could in fact imagine, diagrammatically (as happens in ordinary 
osmotic arrangements) a vertical column of vessels, each provided, say, with 
a short vertical side-tube communicating with the open air, in which the 
pressure is adjusted from moment to moment, and yet such that the sap 
slowly travels by transpiration from each vessel to the one next above, 
* © Roy. Soc. Proc.,’ vol. 74,'p. 554; § Phil. Trans.,’ B, vol. 198, p. 41. 
