To remove the conditions favorable to the development of many kinds of insects.

To sink surface stones so that they will not interfere with harvesting the crop.

Light porous soils may be rolled at any time, but clay soils can be rolled to advantage only when they are stiff and cloddy.

Spring-sown grain is often rolled as soon as sown.  This is all right in ordinary spring weather, but if showers are frequent and the soil is quite moist the rolling should be omitted till after the grain is up.  The same practice will apply to autumn-sown grain also.  If the soil is dry the rolling helps it to pump water up to the seeds.  But if it is moist and showers are frequent the combined action of the roller and the rain is to make so thick a crust that many of the seeds will not be able to force their way through it or will be smothered by poor ventilation.  After the grain is up the rolling may be done to advantage, as it then makes a firm soil about the roots of the plants, a condition of benefit to grain crops.

The most simple form of roller is a solid or hollow cylinder of wood fastened into a frame by which it is drawn.  Some rollers have spikes or blunt attachments fastened to their surfaces for breaking clods.  A roller that is quite popular consists of a cylinder of pressed steel.

CHAPTER XIII

LEAVES

FACTS ABOUT LEAVES

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.