[Illustration:  Fig. 12.  To show where growth in length of the root takes place.  Forty hours before the photograph was taken the tip of the root was 1/4 inch from the lowest thread.  The glass cover was taken from this in order to get a good picture of the root.]

[Illustration:  Fig. 13.  Radish seeds sprouted on dark cloth.  To show root hairs.]

[Illustration:  Fig. 14.  To show how water gets into the roots of plants.  Water passed up into the egg through the skin, or membrane, and forced the contents up the glass tube until it began to overflow.]

[Illustration:  Fig. 15.  To show osmose (see page 19).]

CHAPTER III

SOILS

The soil considered agriculturally, is that part of the earth’s crust which is occupied by the roots of plants and from which they absorb food and moisture.

RELATION OF SOIL TO PLANTS

We have learned that plant roots penetrate the soil to hold the plant in a firm and stable position, to absorb moisture and with it plant food.  We learned also that for roots to do these things well, the soil in which they grow must be mellow and firm, and must contain moisture and plant food, air must circulate in its pores and it must be warm.

How can we bring about these conditions?  To answer this question intelligently it will be necessary for us to study the soil to find out something about its structure, its composition, its characteristics; also, how it was made and what forces or agencies were active in making it.  Are these forces acting on the soil at the present time?  Do they have any influence over the conditions which are favorable or unfavorable to plant growth?  If so, can we control them in their action for the benefit or injury of plants?

We will begin this soil study with an excursion and a few experiments.

Go to the field.  Examine the soil in the holes dug for the root lessons, noticing the difference between the upper or surface soil and the under or subsoil.  Examine as many kinds of surface soils and subsoils as possible, also decayed leaf mould, the black soil of the woods, etc.  If there are in the neighborhood any exposed embankments where a road has been cut through a hill, or where a river or the sea water has cut into a bank of soil, visit them and examine the exposed soils.

=Experiment.=—­Place in separate pans, dishes, plates, boxes, or on boards, one or two pints each of sand, clay, decayed vegetable matter or leaf mould or woods soil, and garden soil.  The soil should be fresh from the field.  Examine the sand, clay and leaf mould, comparing them as to color; are they light or dark, are they moist or not?  Test the soils for comparative size of particles by rubbing between the fingers (Fig. 19), noticing if they are coarse or fine, and for stickiness by squeezing in the hand and noting whether or not they easily crumble afterwards.