"We pass on now to Fig. 93. Here we find a different state of matters. The canals are open and freely supplied with air, while the pores are filled with water; and consequently you perceive that, while the seed _a_ has quite enough of air from the canals, it can never be without moisture, as every particle of soil which touches it, is well supplied with this necessary ingredient. This, then, is the proper condition of soil for germination, and in fact for every period of the plant's development; and this condition occurs when soil is _moist_ but not _wet_--that is to say, when it has the color and appearance of being well watered, but when it is still capable of being crumbled to pieces by the hands, without any of its particles adhering together in the familiar form of mud. "Turning our eyes to Fig. 94, we observe still another condition of soil. In this instance, as far as _water_ is concerned, the soil is in its healthy condition--it is moist, but not wet, the pores alone being filled with water. But where are the canals? We see them in a few places, but in by far the greater part of the soil none are to be perceived; this is owing to the particles of soil having adhered together, and thus so far obliterated the interstitial canals, that they appear only like pores. This is the state of matters in every _clod of earth_, _b_; and you will at once perceive, on comparing it with _c_, which represents a stone, that these two differ only in possessing a few pores, which latter, while they may form a reservoir for moisture, can never act as vehicles for the _food_ of plants, as the roots are not capable of extending their fibres into the interior of a clod, but are at all times confined to the interstitial canals. "With these four conditions before us, let us endeavor to apply them _practically_ to ascertain when they occur in our fields, and how those which are injurious may be obviated. "The first of them, we perceive, is a state of too great dryness, _a very rare_ condition, in this climate at least; in fact, the only case in which it is likely to occur is in very coarse sands, where the soil, being chiefly made up of pure sand and particles of flinty matter, contains comparatively much fewer pores; and, from the large size of the individual particles, assisted by their irregularity, the canals are wider, the circulation of air freer, and, consequently, the whole is much more easily dried. When this state of matters exists, the best treatment is to leave all the stones which occur on the surface of the field, as they cast shades, and thereby prevent or retard the evaporation of water.