190 
NATURE NOTES 
grows in a flower pot of common earthenware, standing in a 
saucer. I constantly pour water into the saucer, supposing 
that it soaks through the flower pot, and so nourishes the roots 
of the ivy. It is quite certain that the air in which this ivy 
lives and thrives is much drier than the outer air ; which seems 
to make it improbable that the leaves absorb and ‘ elaborate 
sap from the atmosphere,’ as suggested in this cutting. I am 
further perplexed by Mr. A. P. Reeves’s statement as to the 
effect of ‘ ringing ’ a tree. I constantly pass some holly trees 
which were ruined years ago by horses gnawing the bark. The 
tops of the trees are dead, but there is a fairly strong growth 
below the ring. Then, when trees are topped or pollarded, or 
(as with Spanish chestnuts in hop-growing districts) cut off 
level, or nearly so, with the ground, why, on Mr. Reeves’s theory, 
do we find that the roots and the lower part of the tree do not 
die, but that a vigorous outgrowth follows ? Surely Mr. Reeves’s 
words, as quoted, are too sweeping.” 
The subject has been complicated by guesses based upon the 
misleading analogy of animal circulation. Plants have neither 
a heart, or central pumping station, nor anything exactly com- 
parable to arterial and venous blood, or the pulmonary purifi- 
cation of the latter by oxygen ; nor is there any true circulation of 
any fluid. Under the term “ sap ” two differing fluids are con- 
fused. Plants undoubtedly, as a rule — excluding, for instance, 
submerged species — depend for water and its dissolved nitrogen-, 
and other, salts upon their roots. This “ unelaborated sap ” 
undoubtedly rises, receiving — as was long ago shown by the 
experiment of a vine cut down to the ground in spring — a powerful 
initial upward impetus from “ root-pressure ” alone. It is 
equally certain that this unelaborated sap ascends through the 
vascular tissue of the sap-wood, and that its ascent is, therefore, 
in no way affected by any “ ringing ” of the bark which does 
not interrupt the continuity of this wood. It is equally certain 
from copious observation and experiment that the plant is 
dependent for the carbon of its food upon the air ; that this 
is taken in by the leaves and other green parts, and is chemically 
combined, to form carbo-hydrates, and probably proteids also, 
with the ingredients of the “ imelaborated sap,” in the leaves 
and growing parts of the shoot ; and that the resultant 
“ elaborated sap ” travels upwards or downwards wherever 
it is required. This “elaborated sap” may pass upwards 
to a growing point or a seed in process of development, 
or it may pass downward to the cambium or growing 
layer of the stem. To reach these destinations it travels 
generally by way of the sieve-tubes in the bast or inner 
bark, so that its downward course will be interrupted by 
ringing, though it may travel in part by the laticiferous 
system, and this is sometimes partly in the pith, and therefore 
