432 THE HOME, FARM AXD BUSINESS CYCLOPAEDIA. 



GROWTH OF ROOTS. The growing point of a root, so far as its length 

 is concerned, is comprised within a small area just above the extreme tip> 

 the extreme tip itself being covered by a little cap shed off from the skin 

 of the root, and serving as a shield to it in its progress through the soil. 



GROWTH OF THE STEM. In the case of the stem and branches, the 

 growing points, by whose agency increase in length takes place, are placed 

 at the summit of the stem, or of its subdivisions, the branches. The 

 growing points then form the substance of the " buds," which are either 

 invested by leaf-scales as protectors and stores of nourishment, as in the 

 case of bulb-scales, or by perfect leaves. The increase in the thickness of 

 stems takes place also by means of the growing tissue, or cambium, the 

 situation of which is different in the two main groups of " Exogens " and 

 " Endogens." To the former series belong all our trees and shrubs, the 

 clover, beet, turnips ; in the latter are the cereals and grasses. 



GROWTH OF LEAVES. The growing points of leaves occur in various 

 situations, according to the kind of leaf. Sometimes, and more generally, 

 the direction of principal growth is from within outward that is to say, 

 from the centre outward (centrifugal) ; in other cases the general tendency 

 is in the opposite direction (centripetal). 



THE ACTION OF MOISTURE ON ROOTS. Much more obvious to the gen- 

 eral observer is the action of moisture on roots. The distance to which 

 roots will travel in search, as it were, of water, and the way in which 

 luxuriant growth and intricate ramification are promoted, when access to 

 it is obtained, are familiar facts. Too frequently drain pipes get choked 

 with a mass of roots whose structure has been changed, and whose exces- 

 sive growth has been stimulated by the presence of copious supplies of 

 moisture. If there is an equal supply of water all round, the growth of 

 the roots will be uniform ; but if, as is more often the case, there is more 

 water on one side than on the other, then the root will curve to the side 

 where there is the fullest supply, and the power thus exerted to get at the 

 water is greater than that of gravity. 



THE ACTION OF LIGHT AND HEAT ON ROOTS. The direct action of light 

 upon roots is, of course, usually of a negative character. The form and 

 direction of growth in the root may, however, be affected by differences of 

 temperature, experienced now on one side, now on another. Darwin has 

 shown that the movements of roots, due to irritation or contact, are checked 

 by too high or too low a temperature. During their passage through the 

 soil, the roots must be constantly subjected to variations of temperature, 

 first on one side and then on another, the variations giving rise to some of 

 the curvatures and bends of the rootlets. 



