What are the reasons for these facts?

A study of the work of the leaves and the conditions necessary for them to perform their work will help us to answer this question.

THE USES OF LEAVES TO PLANTS

=Experiment.=—­(See Fig. 59).  Take a pot or tumbler in which a young plant is growing, also a piece of pasteboard large enough to cover the top of the pot; cut a slit from the edge to the centre of the pasteboard, then place it on the top of the pot, letting the plant enter the slit.  Now close the slit with wax or tallow, making it perfectly tight about the stem.  If the plant is not too large, invert a tumbler over it, letting the edge of the tumbler rest on the pasteboard; if a tumbler is not large enough use a glass jar.  If a potted plant is not convenient a slip or a seedling bean or pea placed in a tumbler of water will serve the purpose.  Prepare several and place some in a sunny window and leave others in the room where it is darker, and observe them from time to time.  In the case of those plants that were set in the sunny window moisture will be seen collecting on the inner surface of the tumbler.  Where does this come from?  It is absorbed from the soil by the roots and is sent with its load of dissolved plant food up through the stems to the leaves.  There most of the water is passed from the leaves to the air and is condensed on the sides of the glass.  A work of leaves then is to throw off or to transpire moisture and thus make room for a new supply of food-laden moisture.  This water is thrown off through little pores or mouths or stomata which are very small and very numerous on the under side of the leaf.  It will be noticed that the plant not placed in the sunlight transpires very little moisture, showing that sunlight helps the leaves in this work of transpiration.

How much water does a plant transpire or throw off from its leaves?

=Experiment.=—­(See Fig. 60).  Fill a common quart fruit jar or can with soil and plant in it a kernel of corn, a bean, a cotton seed or seed of some other plant.  After the plant has grown to be twelve or fifteen inches high, cut a piece of pasteboard a little larger than the top of the jar, cut a hole in the centre as large as the stem of the plant and make a slit from edge to centre.  Soak the pasteboard in melted wax or paraffine candle.  Cool it and then place it over the jar, slipping it around the plant stem.  Now solder the pasteboard to the jar with melted candle making the joints tight all the way around.  Then close up the slit and the hole about the stem.  The jar is now completely sealed and there is no way for water to escape except through the plant.  The plant should be well watered before the jar is closed.  Now weigh the jar and set in the sunlight.  Weigh again the next day.  The difference in the two weights will represent the amount of water transpired by the plant.  The weighings may be repeated until moisture gives out.