XVI 
INTRODUCTION. 
48. When such a stem lasts through the second year, it forms a second 
layer of wood, like the firsjt, deposited between it and the bark, so as to in¬ 
close the former (while a new portion is also added to the inside of the 
liber); and this is repeated year by year as long as the stem lives and 
grows. T he cross-section of such a stem, therefore, exhibits as many con¬ 
centric rings of wood as the stem is years old at the point where the sec¬ 
tion is made. The annual layers being successively added to the exterior 
of those already existent, this mode of growth or structure is said to be 
Exogenous (increasing from without), and plants of the class are named Ex¬ 
ogenous Plants, or Exogens (viz. outside growers). 
49. The bark of an Exogen is readily separable from the wood (there 
being no intermingling of the fibres of the two), more especially at the 
time the new growth is commencing in shrubs and trees, when an organ- 
izable mucilage (the cambium) is poured out between them, in which the 
new cells and woody tubes are developed. Into this the medullary rays of 
the preceding layer extend horizontally, and new ones are also formed, 
while the woody fibres and vessels are vertically interwoven. 
50. The newer woody layers, through which the sap principally rises, 
constitute the sap-wood or alburnum (so named from its white or light 
color). As they become older, their tubes are gradually thickened, and 
the calibre diminished or even almost obliterated by the deposition of solid 
matter within, both of earthy materials derived from the soil, and of peculiar 
organic products, giving to the old wood of each species its peculiar color. 
This old wood is called the heart-wood, or (from its greater solidity) 
the duramen. 
51. The wood of the Pine and its allies (Coniferae, p. 438) is very homo¬ 
geneous, on account of the nearly total absence of ducts or other vessels 
(except in the first layer); and the woody tubes, which are of unusually 
large size, are marked with a row of circular disks (which are spots where 
the wall is thinner) along two of their sides. 
52. In the second plan (4fi), the woody system is collected into separate 
threads (or bundles of woody fibre and vessels), which are distributed 
vertically and irregularly through the whole extent of the cellular sys¬ 
tem ; so that there is no central pith free from woody matter in the centre, 
no distinct bark at the surface, and nothing like a regular layer or hollow 
cylinder of wood between the two. But the cross-section, at the close of 
the first year’s growth, shows the cut ends of the woody threads in the 
form of dots, interspersed throughout the uniform cellular tissue without 
apparent order, although commonly most crowded towards the circumfer¬ 
ence. In a longitudinal section these threads are distinctly traceable 
downwards along the stem, perhaps nearly parallel for a considerable dis¬ 
tance ; but sooner or later curving outwards more or less gradually, and ta¬ 
pering as they descend, they will be found most of them to terminate ob¬ 
liquely in the false-bark or rind. This shows why the bark of such stems 
is inseparable from the wood without laceration. Traced upwards, these 
threads are found to lose themselves in the frame-work of the leaves. A 
Palm-stem and a stem of the Greenbrier (p. 485) are woody examples of 
this sort. An Asparagus-shoot and a stalk of Indian Corn furnish fine 
herbaceous illustrations. Nor do such stems form concentric layers when 
they last from year to year; but the new woody threads as they descend 
are scattered among the others ; chiefly, however, towards the centre, 
over which the terminal bud of the simple stem is directly situated, and 
where they find most room. Accordingly, contrary to the exogenous stem, 
the newest and softest wood is found towards the centre, w’hile the oldest 
and hardest occupies the circumference. This mode of growth or struc¬ 
ture is, therefore, said to be Endogenous (increasing from within), and the 
plants which present it are termed Endogenous Plants, or Endogens 
(viz. inside growers). 
53. The hollow stem (culm, 29) which most Grasses exhibit was origi- 
