26 
that direction where food is most abundant or easily got at. Let one 
examine the roots of a tree growing on the banks of a stream, and see 
what a leash of fine root-threads are produced if the main roots happen 
to be immersed in the water. In like manner the growth and 
lengthening of the shoots, and the swaying to and fro of the branches, 
bring the leaves into contact with gaseous food, and enable them to 
avail themselves of it without necessitating the movement of the whole 
plant from place to place in search of nourishment, as is imperative in 
the ease of most animals. 
“The roots and the leaves are the chief, and, in many cases, the 
only feeding organs of the plant. The roots imbibe water from the 
soil by means of fine fibrils and root-hairs, the older, thicker portions 
having no such faculty of absorption, but serving merely as conduits 
and holdfasts. The water which exists in and amongst the particles 
of the soil dissolves certain of its ingredients, so that when it enters 
the roots it is not absolutely pure, but holds in solution a small 
quantity of gaseous as well as of earthy or mineral substances. These 
are required in the building up of the plant’s substance, and in the 
formation of its secretions. The wayi?) which this solution or earthy 
and gaseous matter is absorbed into the tissues of the i*oots has now 
to be explained. It has been shown that, when a bladder containing 
some tliick liquid, such as syrup, is placed in a vessel of some thinner 
fluid, such as water, there is a passage of the thinner liquid through 
the membrane into the interior, so that the thick liquid becomes 
diluted and the bladder stretched. Tliis is precisely what takes place 
in the ease of the roots. The thin solution of earthy matter pa.sses 
through the membranous walls of the root cells, there to miugle with 
the thicker protoplasm which they contain. This process of absorption 
is technically called osmo!fis% or endosmosis. 
“ lloot-absorption is probably ahvaj^s going on more or less, but 
it is infinitely more rapid and abundant when a plant is in full growth. 
The fluid wheu absorbed by the roots receives the name of ‘ sap.’ 
"We know, by observation and experiment, that this sap rises from 
the root, passes up the stems, through the branches, and enters the 
leaves. The sap, then, flows upwards, and it is a matter of great 
interest to ascertain how it is that such a fluid should ascend against 
gravity.” [No thoroughly satisfactory solution of the problem has 
yet been arrived at. — “ The explanations are manifold — 
several causes co-operate to bring about the result. In the first 
place, the process of osmosis begun in the root-cells, is continued in 
the young portions of the stem. Moreover, there now comes into 
operation a process of di:ffusion, by "virtue of which certain liquids pass 
through others. G-nihaiu, an English chemist, called the thin, readily 
diifusible liquids, ‘crystalloids,’ the thicker, less easily diffused fluid, 
‘ colloids,’ from their gluey or gummy nature; and he demonstrated 
that the crystalloid fluids pass through and diffuse themselves amongst 
the colloid ones. When the leaves are fully expanded another 
circumstance helps powerfully to promote the rise of the sap, and this 
is the profuse perspiration or evaporation of watery vapour and fluid 
from their surface. Let a few leaves be gathered and placed under a 
tumbler exposed to the sun, and shortly will be seen a quantity 
of water condensed on the sides of the tumbler, which lias been 
evaporated from the leaves. This outflow takes place to an enormous 
