_Demonstration._--Water given off by plant in sunlight. Loss of weight due to transpiration measured. _Laboratory exercise._-- (_a_) Gross structure of a leaf. (_b_) Study of stoma and lower epidermis under microscope. (_c_) Study of cross section to show cells and air spaces. _Demonstration._--Reaction of leaves to light. _Demonstration._--Light necessary to starch making. _Demonstration._--Air necessary to starch making. _Demonstration._--Oxygen a by-product of starch making.
What becomes of the Water taken in by the Roots?--We have seen that more than pure water has been absorbed through the root hairs into the roots. What becomes of this water and the other substances that have been absorbed? This question may be partly answered by the following experiments.
[Illustration: Apple twigs split to show the course of colored water up the stem.]
Passage of Fluids up the Stem.--If any young growing shoots (young seedlings of corn or pea, or the older stems of garden balsam, touch-me-not, or sunflower) are placed in red ink (eosin), and left in the sun for a few hours, the red ink will be found to have passed up the stem. If such stems were examined carefully, it would be seen that the colored fluid is confined to collections of woody tubes immediately under the inner bark. Water evidently rises in that part of the stem we call the wood.
[Illustration: Experiment to prove that water is given off through the leaves of a green plant.]
Water given off by Evaporation from Leaves.--Take some well-watered potted green plant, as a geranium or hydrangea, cover the pot with sheet rubber, fastening the rubber close to the stem of the plant. Next weigh the plant with the pot. Then cover it with a tall bell jar and place the apparatus in the sun. In a few minutes drops of moisture are seen to gather on the _inside_ of the jar. If we now weigh the potted plant, we find it weighs less than before. Obviously the loss comes from the water lost, and evidently this water escapes as vapor from either the stem or leaves.
[Illustration: The skeleton of a leaf. _M.R._, the midrib; _P._, the leafstalk; _V._, the veins.]
The Structure of a Leaf.--In the experiment with the red ink mentioned above we will find that the fluid has gone out into the skeleton or framework of the leaf. Let us now examine a leaf more carefully. It shows usually (1) a flat, broad _blade_, which may take almost any conceivable shape; (2) a _stem_ which spreads out in the blade (3) in a number of _veins_.