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 si 
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 cireum- 
stance 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 un- 
practised 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 
