PHYSIOLOGY 
culties are directly traceable to over-watering, or "cold 
feet," the effect of too much water being partially to 
prevent aeration. Water plants have adapted them- 
selves to getting oxygen in other ways, and many bog 
plants send to the surface special roots for aerating 
purposes. 
No plant can live without oxygen. In some way or 
other oxygen must be secured. The more active a plant 
is, whether in growth or in movement, the more oxygen 
will it require. Even dry seeds must respire slightly, 
and in some kinds respiration may be so rapid that after 
a single season death may ensue. This use of oxygen, 
whether by the germinating seed or by the growing 
or assimilating part, is accompanied by the giving off 
of carbon diOxid, or foul gas. This whole process is 
respiration; and in its ultimate effects it is similar to 
respiration in animals. 
The role of Water in the plant.-even quicker to 
manifest itself than the action of suffocation by lack of 
oxygen is the injury which most plants may suffer from 
an insufficiency of water. The rigidity of an herbaceous 
or succulent plant is due largely to its water content; 
and without a substantial degree of this rigidity, growth 
would cease and life soon become extinct. The plant 
pulled up by its roots or cut down, wilts almost imme- 
diately. The wilting of plants, then, is due to a lack or 
loss of water supply. 
The way in which the ordinary plant may constantly 
obtain a quantity of water from the soil is worthy of 
full discussion. On pulling from the soil a growing 
plantlet of squash, we find a tap-root and a number of 
small rootlets. To the latter cling, perhaps, small par- 
ticles of the soil, as in Fig. 1777. If, however, seeds 
are germinated between pieces of moist paper or cloth 
so that there will be no disturbance of the delicate grow- 
ing parts, further structures will be evident. From a 
quarter of an inch or so behind the root-tip, and ex- 
tending backward for a considerable distance, the root- 
lets are clothed with numerous delicate hairs (shown in 
Figs 1778 and 1788). These are the root-hairs, and it was 
to such as these that the soil clung in Fig. 1777. They 
are simple, long, tube-like cells consisting of a cell wall 
with living protoplasm and cell sap. The inner proto- 
plasmic lining of this cell wall permits water and salts 
in solution to pass inward by the interesting process 
of osmosis. 
The root-hairs are temporary structures which never 
grow into rootlets, but which die away as the roots 
become old or woody. while living they perform the 
important function of absorbing from the soil nearly all 
of the water needed by the plant. Being numerous and 
extremely delicate, they come into the closest touch with 
the surface film of water adhering to the little particles 
of soil, and from such film water they more readily 
satisfy their needs than from free soil water. They 
can extract water until the soil contains only a very 
small per cent, or until it is dust-dry. 
The root-hairs absorb water freely, and during active 
growth it is forced upward into root and stem so vigor- 
ously that a pressure (root pressure) of considerable 
extent may be manifest. If the plant be severed and a 
tube applied to the stump, this pressure manifests itself 
by lifting a column of the liquid absorbed, and often to a 
considerable height. In any herbaceous plant it may be 
tested, as in Fig. 1779. In the grape vine 36 ft. of water 
may be maintained. The bleeding of plants is an evi- 
dence of root pressure. 
Water is actually absorbed in much greater quantity 
than is required merely as a constituent of the plant 
body. In fact, to form one ounce of plant substance it 
is estimated that 15-25 pounds of water must pass 
through the plant. This surplus water passes off 
through the leaves and other succulent parts, princi- 
pally through the stomata previously mentioned. This 
process is one of evaporation from living membranes, 
and it is called transpiration. That transpiration is not 
merely an evaporation process may be roughly shown 
by an experiment with two similar leafy branches 
freshly severed. One of these is dipped in hot water 
to kill the protoplasm, then the two are left to dry out. 
Transpiration from the living twig will be less rapid 
than evaporation from the dead one. The demonstra- 
tion of transpiration is an easy matter. A leafy branch 
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of any plant may be cut off and the end inserted through 
a bored cork into a bottle of water. Over the whole may 
be placed a larger jar or bell-glass, and in a short time 
a mist will collect on the inside walls of the latter. 
Transpiration is facilitated by dry air, wind, high tem- 
peratures, movement of the plant, etc. If on a hot day 
or in dry weather transpiration is greater than the 
amount of water absorbed by the roots, the plant wilts. 
A very slight shower will refresh a wilted plant, but 
not because the leaves have absorbed water. The plant 
recovers because the air is saturated, and transpiration 
is thereby so much lessened that the roots can catch up 
in furnishing the necessary supply. 
Fully a quart of water is daily transpired from a form- 
ing cabbage head, and the number of tons of water daily 
given off per acre by forest or even meadow-land may 
reach an astonishing figure. The amount of water tran- 
spired by a small potted plant may be readily weighed. 
Place the pot in a glass jar as seen in Fig. 1780, tying 
over the top and snugly around the plant some soft rub- 
ber cloth. water the plant through the glass tube and 
then weigh. After standing 6 or 12 hours in a dry 
atmosphere the weight will be consid- 
• - , erably reduced, due to the loss by 
transpiration. 
The path of the ascending water 
current or sap current is through par- 
ticular vessels of the young woody 
parts. In trees it ascends in the 
youngest wood rings, not between bark 
and wood In herbaceous netted-veined 
(dicotyledonous) plants the path is in 
the ring of woody tissue or bundles 
between the bark and pith. In the 
Indian corn (monocotyledonous) it is 
in the thread-like groups of fibers 
(fibrovascular bundles ) scattered rather 
irregularly throughout the pith. That 
the current is always through these 
1779. 
To test root pressure, 
woody bundles in the above plants may be demonstrated 
by placing branches of the plants in a tumbler contain- 
ing some eosin solution. In a few hours the bundles 
will be colored for a considerable extent The current 
will rise much faster if the branch to be used is cut un- 
der water. This prevents the access of air and the par- 
tial stoppage of the conducting channels. For the same 
reason flowers wilt less rapidly when the stems are cut 
under water. . 
The total amount of water in plants varies from a very 
small percentage in dry seeds to about 50 per cent in 
