iv
The manner in which the fibres descend from the buds through the stem differs in different classes of vegetables, and two of these modes require a short notice, as a most important division of the flowering plants is partly founded upon them. If we examine the stem of a Palm when cut transversely, we shall find it presents a dotted appearance, as in Fig. 1, the dots being sections of the vessels descending from the leaves, in their way to the root. There is no distinct bark, the stem being covered by a mere rind or cuticle; and the wood itself is the same throughout, the fibres being only rather closer together towards the outer portion. Stems of this class are called endogenous, or inward-growing. If we take a section of the branch of an Oak, or any of our forest trees, we find a very different arrangement. The wood presents a series of concentric circles surrounding a central column of pith, from which numerous rays diverge towards the bark, as in Fig. 2. In stems of this kind the fibres or bud-rootlets descend each year on the outside of the previously formed wood, between the latter and the bark, and thus each circle represents the annual growth of the stem. The bark is distinct and highly organized, consisting of an inner portion, formed of vessels descending from the leaves and carrying off their secretions, and an outer mass made up of the growth of previous years, filled up with cellular tissue. Stems presenting this aspect are termed exogenous, or outward-growing. These forms characterize the two great families or classes into which the flowering plants are divided, named from the circumstance Endogens and Exogens. The endogenous plants of this country have nearly all annual stems, and most of the exogenous ones being likewise herbaceous, it is not always easy for the unpractised observer to distinguish the difference in their internal structure; but there are other peculiarities by which they are more readily recognized, as will be presently noticed.
The leaves are the breathing organs of the plant, and, like the lungs of an animal, they are formed of a number of vessels
