ABSORPTION 
sibly thicken it, but which exerts a noxious in- 
fluence on vegetable growth, it will absorb a full 
proportion of the accompanying ingredient ; and 
if placed in water mixed with a substance which 
very sensibly thickens it, yet which exerts a nu- 
tritive and benign influence on the vegetable 
economy, it will refuse to take up a fair propor- 
tion of the accompanying ingredient. Plants, 
for example, which are placed in water holding 
in solution some sulphate of copper,—a substance 
of very noxious character,—will absorb the liquid 
almost as freely as if it were pute water; plants 
which are placed in solutions of gum, of various 
proportions of thickness or viscosity, will absorb 
some proportion, though not a due one, of the 
least viscous solution,—and will absorb a less 
proportion of the more viscous solution,—and 
will absolutely perish in such solutions as are 
highly viscous; and plants which are placed in 
the drainings of dunghills, or in such waters as 
hold in solution or suspension the liquefied or 
minutely divided product of the decomposition 
of organic substances, absorb smaller quantities 
of these liquids than they would of pure water, 
and, at the same time, derive from them a larger 
| proportion of the elements of nutrition and 
growth. The spongioles thus appear to be con-_ 
trolled, to a great degree, by the mechanical con- 
dition of the liquids mixed with the soil; and are 
probably obstructed, by various substances, espe- 
cially by such as are viscous, in the pores, cells, 
or passages of their exceedingly minute tubes. 
One experimenter, however—M. Pollini—found 
that plants which he used absorbed different 
kinds of solutions without much seeming regard 
to their respective degrees of viscosity,—and that 
they absorbed more of sugar than of gum, and 
more of potass or of common salt than of the 
acetate or the nitrate of lime. This experimenter 
found also that, when the extremities of the roots 
were cut away, so as to divest a plant of its spon- 
gioles, and when the mere stumps of the roots or 
horizontal sections of the absorbing tubes were 
plunged in various solutions, absorption ceased 
_ to be modified by the same mechanical conditions 
as before, and took up indifferently all the kinds 
of salts which the several solutions contained. 
The spongioles of one set of plants would, in con- 
sequence, seem to be very differently organized 
.| from the spongioles of other sets of plants,—those 
of some more readily admitting certain sub- 
stances than those of others,—those of the smaller 
proportion of plants affected only in a small de- 
gree by the mechanical condition of liquids,—and 
those of the larger proportion affected, for the 
‘most part, in a very great degree ; and the spon- 
gioles, in all cases—though far more markedly 
in the latter cases than in the former—would 
seem to afford a far more qualified, hesitating, 
and elective admission of many substances in 
solution than is temporarily afforded by the hori- 
zontal sections of the absorbing tubes. A plant 
can live only two or three days in a solution of | sorbing power of the leaf, however, does not re- 
IN PLANTS. 
sulphate of copper, but will live eight or ten days 
in a solution of gum; yet, as has been already 
stated, it absorbs comparatively much of the sul- 
phate, and comparatively little of the gum. 
The absorbing power of the leaves of plants was 
long a more obscure topic than the absorbing 
power of the roots; but it was investigated with 
considerable success by the naturalists, Duhamel 
and Marriotte, and very distinctly ascertained by 
the experiments of M. Bonnet of Geneva. The 
main object of M. Bonnet’s experiments assumed. 
that a leaf possesses an absorbing power, and 
sought to determine whether it exerts this power 
alike on both of its surfaces. ‘ With this view,” 
says Keith, “he placed a number of leaves over 
water, so as that they floated on it but were not 
immersed; some with the upper surface, and 
others with the under surface applied to the 
water. If the leaf retained its verdure longest 
with the upper surface on the water, the absorb- 
ing power of the upper surface was to be regarded 
as the greatest ; but if it retained its verdure the 
longest with the under surface on the water, then | 
the absorbing power of the under surface was to 
be regarded as the greatest. Some leaves were 
found to retain their verdure the longest when 
moistened by the upper surface, and some when 
moistened by the under surface; and some were 
altogether indifferent to the mode in which they 
were applied to the water. But the inference 
deducible from the whole, and deduced accord- 
ingly by Bonnet, was, that the leaves of herbs ab- 
sorbed moisture chiefly by the upper surface, and 
the leaves of trees chiefly by the under surface. 
What is the cause of this singular disparity be- 
tween the absorbing surfaces of the leaf of the | 
herb, and of the tree? The physical cause might 
be the existence of a greater or of a smaller 
number of pores found in the leaves of the herb | 
and tree respectively. The chemical cause would | 
be the peculiar degree of affinity existing be- 
tween the absorbing organs and the fluid ab- 
sorbed. Duhamel seems to have been content to 
look to the physical cause merely, regarding the 
lower surface of the leaf of the tree as being en- 
dowed with the greater capacity of absorbing 
moisture, chiefly for the purpose of catching the 
ascending exhalations which must necessarily 
come in contact with it as they rise, but which 
might possibly have escaped it if absorbable only 
by the upper surface, owing to the increased 
rapidity of their ascent at an increased eleva- 
tion ; and regarding the upper surface of the leaf 
of the herb as being endowed with the greater 
absorbing power, owing to its low stature, and 
to the slow ascent of exhalations near the earth. 
This did not throw much light upon the subject ; 
and the experiments were still deemed insuffi- 
cient, as not representing to us the actual pheno- 
mena of vegetation, though the fact of the ab- 
sorption of moisture by the surface of the leaf is 
fully confirmed by such phenomena.” ‘The ab- 
“|| 
