260 THE MICROSCOPICAL NEWS. 
altering its form. It is certain that the elements of which wood is 
formed are undergoing ceaseless alteration during the growth of the 
wood before combustion re-converts the wood into those elements ; 
it is equally certain that the heat which is absorbed by plants is 
doing ceaseless work before it reappears as heat when the com- 
bustion takes place. Nothing in nature is useless, nothing in nature 
is still. 
The chemistry and the physics of plant-life at no time more 
forcibly and unitedly arrest the attention than when one considers 
the character of the watery fluid or sap of plants, what it is, its 
functions, and how it performs those functions. For in the sap of 
the plant-cells the water itself and the mineral matters brought up 
from the roots, and the gases brought in from the air, meet and 
sunite. In the sap is commenced the structure which when per- 
fected forms the leaf, the flower, the fruit, the whole edifice of 
usefulness and beauty. 
Now, knowing something of chemistry and of physics myself, 
and having never lost my love for botany since the pleasant days 
of my studentship in that science some thirty years ago, under my 
present colleague Robert Bentley, I recently felt my interest in sap 
strongly aroused by the sight of some literally raining on me from 
a wounded tree growing in my own garden at Watford. 
On the evening of the 3rd of April, beneath a white birch I 
noticed a very wet place on the gravel path, the water of which 
was obviously being fed by the cut extremity of a branch of the 
birch about an inch in diameter and some ten feet from the 
ground. I afterwards found that exactly fifteen days previously, 
namely; on March roth, circumstances rendered necessary the 
removal of the portion of the bough which hung over the path, 4 
or 5 feet being still left on the tree. The water or sap was dropping 
fast from the branch, at the rate of 16 large drops per minute, each 
drop twice or thrice ‘the size of a “minim.” Neither the catkins 
nor the leaves of the tree had yet expanded. I at once decided 
that some interest would attach to a determination both of the rate 
of flow of the fluid and its chemical composition, especially at such 
a stage of the tree’s life. For although a good deal is already 
known respecting the “bleeding” of trees and the general character 
of the exuding fluid, very much remains to be discovered. Indeed, 
I could scarcely myself hope to do more than confirm some 
previous observers and perhaps give quantitative value in just one 
or two directions to the qualitative experiments of others, Thus, 
that the birch readily yields its sap when the wood is wounded is 
well known. Phillips, quoted by Sowerby, says : 
** Even afflictive birch 
Cursed by unlettered youth, distils 
A limpid current from her wounded bark, 
Profuse of nursing sap.” 
