Ings around the stopper and plant be sealed to prevent

evaporation. Since there was no other way of escape

for the water, it must have been taken into the plant and

passed upward through the stem. The process by which

the plant accomplishes osmosis is complex. Liquids

and gases — even solids in some cases — are driven

through membranes or caused to diffuse into one another

by a gigantic force spoken of as osmotic pressure.

When copper sulfate (blue vitriol) crystals are covered

with water, solution begins and a blue color gradually

creeps throughout the water until a uniform color exists.

Samples taken from any part of the vessel would show

nearly equal concentrations of copper sulfate, which, at

the beginning of the experiment, was confined to the

immediate vicinity of the immersed crystals. This

diffusion, or mixing of the salt throughout the water,

was impelled by the power of diffusion or osmotic pressure.

If the vessel containing water be divided by a partition

of parchment or piece of animal bladder, a change in the

final result is apparent. After awhile the water on one

side of the membrane rises and lowers on the other.

Since the membrane admits the free passage of water

but not of salt, fresh water is driven through in an attempt

to make the liquid of uniform concentration. Some salt

passes through into the fresh water, but the chief move-

ment is made by the water.

To demonstrate osmosis, a strong solution of common

salt may be placed in a thistle tube, over the large end of

which a piece of parchment paper or bladder is tied tightly

to shut out air (Fig. 13). If the solution stands high

enough to reach into the small tube above the bulb, a piece

of string or an elastic band can be used to mark the height.

After immersion in fresh water from a few minutes to a

few hours, the solution rises in the tube showing the intake

46 The Principles of Agronomy

of water. Osmosis, agriculturally, is the process by

which water from the dilute solution flows through a

semi-permeable membrane into the more concentrated,

in an attempt to equaUze the strength of the solution.

It is by osmosis and due to osmotic pressure that roots

take in water. Root-hairs contain concentrated solutions

in the cell-sap which set up a difference in osmotic pressure

between the cell and the water outside. Students of

physical chemistry have found that this pressure is enor-

mous, amounting in many cases to tons, and that it in-

creases as the difference in solution-concentrations in-

creases, and aa the temperature rises. So long as the

cell-sap is more concentrated than the soil solution, water

passes inward. If strong solutions are brought in contact

with the root-hairs, osmosis ceases or goes in the opposite

direction and the cells become flabby and wilt. This

is one injury caused by strong alkali.

Plants seem able to exercise a power of selective absorp-

tion ; that is, if salts are not used by the plant, they enter

only in small quantities ; while the useful elements go in

rather freely. This careful adjustment helps to keep out

harmful substances and to take in the raw mineral plant-

foods. Plant cells full of water are rigid and hold their

shape. As one loses water, osmotic pressure causes more

to enter. Throughout the plant there is some move-

ment of water due to osmosis.

34. Transpiration. — Not only do roots take in water

enough to maintain the plant in a rigid condition, but

they must, in addition, maintain a stream that passes

entirely through the plant. Because the water evaporated

from the leaves is in the form of vapor, it cannot be seen

under ordinary conditions. On cool mornings, however,

droplets of moisture are often visible on the surface of

leaves. Water vapor, escaping by means of the stomata,

Plant Functions 47

partly condenses when cooler air is reached. Even on

hot days, in Hving or school rooms, transpiration — as

this giving off of water is called — can be demonstrated

by covering a leafy house-plant such as a geranium with a

clean glass jar or open-

mouthed bottle. In

two or three hours the

transpired water will

collect on the glass in

drops, and under favor-

able conditions with a

healthy plant will drip

down the sides.

Plants transpireenor-

mous quantities. For

each pound of dry sub-

stance they add to their

weight by growth, over