92 THE LEAVES OF PLANTS. 



tree) -will not grow upon the spot where a tree of the same species recently grew ; and 

 that, not because the soil was exhausted, but that a poisonous excrementitious matter 

 had been deposited there by the roots of the former plant. 



There are certain plants which emit their roots above the surface of the soil, as the 

 Pandanus or Screw Pine (Fig. 148), and the Mangrove tree. Such roots are termed 

 aerial roots; since, until they have sufficiently elongated to reach the earth, they remain 

 above the ground. Their natural tendency, however, is to bury themselves in the 

 ground, and they become eventually roots. 



Roots vary greatly in figure, and therefore demand special names ; the most common 

 is the fibrous, as in the oak and forest trees, where the body of the 

 root is divided into many smaller, elongated, conical portions. If 

 there is but one conical elongation, the root is termed fusiform. 

 When the root seems to terminate suddenly at the body, it is termed 

 premorse, or bitten off ; if it be fleshy, and divided into several globose 

 parts, it is known as many-headed, or as tubercles in some of the 

 orchids. 



It is often of importance to distinguish accurately between an 

 underground stem and a root; this is chiefly effected by negative 

 evidences. Thus a root has none of the appendages of the stem, 

 such as leaf buds, leaves, scales, scars, and stomata ; and in exogens 

 it has no pith. Some roots have the power of forming adventitious 

 buds, but the buds never proceed in a regular manner from fixed 

 points. So also with branches, when they occur ; they proceed not 

 Fig 149 A fleshy ^ rom ^ ea ^ buds, but i n an irregular way from any point of the surface, 

 fibrous root. By these various signs we infer that such enlarged parts of plants, 

 S.'Thefibrilftejwith as the potato, turnip, and radish, are true stems, although they 

 the terminal spon- aro situate under- ground, and that they give off the true fibrillse or 



roots from their apices or sides. 



Appendages of the Stem. Under the head of appendages of the stem or axis 

 we shall have to consider the respiratory and reproductive parts of plants, such as the 

 leaves, flowers, and fruit, with their subordinate structures, and shall take them in the 

 order in which they appear upon the stem. 



Leaves. The leaf is the type of construction of all the appendages of the axis, no 

 matter how developed soever may be their external configuration. It is therefore not 

 only imperative to a right understanding of other organs that these should be well 

 studied, but a knowledge of the composition of the leaf is the readiest mode of becoming 

 acquainted with the structure of its prototypes. We will, therefore, invite our readers 

 to bear in mind that the immense variety in the figure of the leaf, and in the leaves 

 and other parts of the flower and fruit, does not imply any difference in structure, lut 

 that a knowledge of one is a knowledge of all. 



The leaf is technically said to be " an expansion of the bark at the base of a leaf- 

 bud ;" but such a definition gives no idea of its structure. A more tangible definition 

 is, that it is a flattened and expanded stem ; for every structure, which enters into the 

 composition of the stem and none other, is present in the leaf. Thus there is cellular 

 and vascular tissue inclosed on each side by a cuticle. 



The leaf is, for the most part, a flattened organ, having two surfaces, or paging a 

 border, a bore, and an apex, the whole of which constitutes the lamina or blade ; and 

 it is connected with the stem by a foot-stalk or petiole. The surface is 



