FUNCTIONS OF THE HOOT MODIFIED BY THE SOIL. 97 



tions, — in the root and in ihe stem as well as in the leaves, — atone time 

 in the dark, at another under the influence of the sun's rays, — exposed 

 when in the leaf to the full action of the air, — and when in the root al- 

 most wholly secluded from its presence ; — the new compounds pro- 

 duced in every instance being suited either to the nature of the plant or 

 the wants and functions of that part of it in which each transformation 

 takes place. 



To some of these transformations it will be necessary to advert more 

 particularly, when we come to consider the special changes by which 

 those substances of which plants chiefly consist, are formed out of these 

 compounds on which they chiefly live. 



§ 7. Circumstances by which the functions of the various parts of plants 

 are modified. 

 Plants grow more or less luxuriantly, and Their several parts are 

 more or less largely developed, in obedience to numerous and varied 

 circumstances. 



I. In regard to the special functions of the ropt, we have already seen 

 that the access of atmospheric air is in some cases indispensable, while 

 in others, by shooting vertically downwards, the roots appear to shun 

 the approach of either air or light. It is obvious also that a certain de- 

 gree of moisture in the soil, and a certain temperature, are necessary 

 to the most healthy discharge of the functions of the root. In hot wea- 

 ther the plant droops, because the roots do not absorb water from the 

 soil with sufficient rapidity. And though it is probable that, at every 

 temperature above that of absolute freezing, the food contained in the 

 soil is absorbed and transmitted more or less slowly to the stem, yet it is 

 well known that a genial warmth in the soil stimulates the roots to in- 

 creased activity. The practice of gardeners in applying bottom heat in 

 the artificial climate of the green-hoase and conservatory is founded on 

 this well-known principle. 



But the nature of the soil in which plants grow has also much influ- 

 ence on the way in which the functions of the root are discharged. As 

 a general fact this also is well known, though the special qualities of the 

 soil on which the greater or less activity of vegetation depends, are far 

 from being generally understood. If the soil contain a sensible quantity 

 of any substance which is noxious to plants, it is plain that their roots 

 will be to a certain degree enfeebled, and their functions in consequence 

 ofily imperfectly discharged. Or if the soil be deficient either in organic 

 food, or in one or other of those inorganic substances which the plants 

 necessarily require for the production of their several parts, the roots 

 cannot perform their office with any degree of efficiency. Where the 

 necessary materials are wanting the builder must cease to work. So in 

 a soil which contains no silica, the grain of wheat may germinate, but 

 he stalk cannot be produced in a natural or healthy state, since silica is 

 indispensable to its healthy construction*. 



II. The ascent of the sap is modified chiefly by the season of the 

 year, by the heat of the day, and by the genus and age of the plant or 

 tree. 



There seems reason to believe that the plant never sleeps, that even 

 during the winter the circulation slowly proceeds, though the first 



