THE PLANT AND ITS PHYSICAL ENVIRONMENT. 205 



which cause the water to be retained on the particles of soil may 

 finally balance the forces which operate in causing the water to enter 

 the root. WTien this point is reached permanent wilting takes place. 

 When the soil forms part of the ground, i.e. is not isolated from the 

 rest of the surrounding soil, water continually flows from soil where 

 it is more loosely held {i.e. is physically more abundant) to soil where 

 it is deficient, till equilibrium is maintained. But this movement of 

 the soil water may be too slow to satisfy the needs of the plant, and 

 then recovery from wilting will not take place. In dry weather it is, 

 as everyone knows, of great advantage to mulch the soil, and the 

 most effective mulch is obtained by keeping the surface continually 

 fined with a hoe. The explanation is to be found in the fact that 

 when the top soil is disintegrated in this way the capillary passages 

 are broken, and a mixture of soil and still air forms a blanket over the 

 unbroken soil in which the water flow is taking place, consequently 

 very little loss of water from the surface of the hoed ground occurs, 

 and only that actually lost by the plants is of real consequence. I have 

 seen a bed of roses, growing on sandy gravel soil, kept perfectly fresh 

 after six weeks' dry weather, during which time not a drop of water had 

 been given by hose or watering-pot. One sees the same sort of thing 

 in a sand dune, where it frequently happens that the sand a few inches 

 below the powdery surface is quite moist — sufficiently so to bind when 

 squeezed in the hand. It is on the application of the foregoing 

 physical facts that what is known as " dry farming " is rendered 

 possible on a commercial scale. Further investigation is, however, 

 needed to ascertain whether, and if so to v/hat extent, the soil is able to 

 increase its supplies of water from the air which enters it during cold 

 nights, when the temperature of the ground may be greatly lowered 

 by radiation. It may well be that some, but probably not all soils, 

 gain supplies of moisture in this way which are large enough to be of 

 material importance. 



The relations of the plant to its physical environment are by no 

 means exhausted by a consideration of some of the temperature and 

 water factors. The gases of the atmosphere, light, and many other 

 things have to be taken into account. We will glance at a few of these 

 other points. 



Everyone knows that the ordinary green plant does not derive 

 its food solely from the earth, but that the carbon dioxide present in 

 the atmosphere forms an essential food material. From this source 

 alone the carbon, which is of such overwhelming importance in the whole 

 of the chemical processes of plants, is obtained. Not only does carbon 

 enter into the food and other essential materials within the plant, but 

 it is an integral constituent of the vital substance itself. The steps 

 by which the carbon dioxide and water taken into the plant undergo 

 those complex changes which culminate in the formation of new living 

 substance are being slowly traced, but the matter is too complex, even 

 did time permit of it, for me to attempt to deal with the many problems 

 which are involved. Suffice it to say that the sugars which are thus 



