342 
* * 
harder, more durable, and generally of a darker color than a few of the outside- 
layers—the alburnum, or sap-wood. The difference is owing to the deposition 
of matter (sclerogen) in the tissue of the heart-wood; by digesting a piece of 
heart-wood in hot nitric acid, the coloring matter may be discharged. When 
the heart-wood is filled with secretions, it no longer performs any useful func¬ 
tion ; hence trees live many years after the centre of the trunk has decayed. 
The structure of the root of an exogen is similar to that of the stem, but 
there is a difference in several important points. The stem has a distinct pith, 
but the root has none; it has, however, a distinct medullary system. In some 
instances, as in the Horse-chesnut, the pith does extend some distance into the 
root. Stems have a provision for a symmetrical arrangement of leaves and 
branches; leaf-buds being placed at regular intervals. No such provision 
occurs in roots; they ramify irregularly, partly according to the nature of the 
plant, and partly according to the nature of the soil. The fibrous roots of 
plants being so constructed as to grow most in that direction wherein they 
meet with most food. Stems in their young state are green, and are provided 
with stomata; subterranean roots are never green, and have no stomata. 
Aerial roots, as those of the highly interesting tribe of epiphytal orchids, 
which grow on the stems of trees in tropical countries, are green and possess 
stomata. Young stems increase or grow throughout their whole length, 
Hoots have to make their way through substances which offer resistance to their 
progress, it is therefore wisely ordained that they shall lengthen exclusively by 
additions to their points. A plant absorbs nourishment from the soil exclu¬ 
sively, by the extremities of the young roots, called spongelets, or spongioles. 
Besides fixing the plant in the soil and absorbing fluids, roots are often reservoirs 
of nutriment stored up for the future wants of the plant; the roots of the 
carrot and turnip, for instance, contain a fund of organizable matter destined 
chiefly to feed the blossoms and seeds of the following summer. 
Buds are of two kinds, leaf-buds and flower-buds. Leaf-buds contain the 
rudiments of branches, and aro formed in the axils of leaves, in the angle 
formed between the stem and leaf. Many leaf-buds remain dormant; they do 
not, under ordinary circumstances, grow into branches. If we cut off the 
branches of a young tree, buds will be irregularly developed on the stem; 
these are called latent or adventitious buds; they are generated by the hori¬ 
zontal or medullary system. In temperate climates buds remain dormant 
during winter, and are protected by scales, which are modified leaves. Some 
