We found in an earlier lesson that all of our farm plants have roots, stems, leaves, flowers, fruit and seeds. We studied the root first as being the most important part of the plant to the farmer. The seed was the next part studied, for that was considered the next most important, because the seed is the main reliance for new plants. The part next in importance is the leaf and that we will now study. If you will go into the field and observe the leaves on a number of plants, you will find that the following facts are true: They are all green. They are flat and thin. Many of them are very broad. Some of the leaves on a single branch are larger than others on the same branch, and some have longer stems than others. Most of them have a rather dark glossy upper surface and a lighter rougher under surface. [Illustration: FIG. 59. To show transpiration. Plant _A_ was set in the sunlight, plant _B_ was left in the darker part of the room. _A_ has transpired much more than _B_, showing that sunlight is necessary for this work.] [Illustration: FIG. 60.--AMOUNT OF TRANSPIRATION This plant transpired within 48 hours an amount of water equal to the colored liquid in the bottle standing on the jar, more than 6 ounces.] The leaves on the lower branches of the trees are spread out in a more or less flat layer and have their glossy surfaces all turned up, while those on branches in the tops of trees or shrubs are arranged all around the branch, the glossy surface being turned up. 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