INTRODUCTION TO BOTANY. XXi 
§ 3. The Root. 
160. Anatomically the root differs from the stem in wanting normally developed 
leaf-buds (29), stomates (158), and in Exogens, a distinct pith. It increases in 
length by constant small additions to its extremity, and thus is enabled to force its 
way through the soil, and to diverge when rocks or obstacles oppose its progress. 
161. The functions of the root are to fix plants in or to the soil or other substance 
on which the plant grows, to absorb nourishment either from the soil, or in the case 
of aerial roots, from the air, and to transmit it rapidly to the stem. Absorbtion 
takes place through the younger fibres or the extremities of the root-branches, and 
through hairs which are formed on all young roots, when growing vigorously. The 
nutriment thus absorbed consists chiefly of carbonic acid and nitrogen, or nitroge- 
nous compounds, dissolved in water. 
162. Parasites are plants whose roots bury themselves in the cellular structure of 
other plants, and subsist on nourishment sucked out of the plant which they attack. 
Epiphytes are those whose roots attach themselves to other plants without penetra- 
ting into their cellular tissue. 
§ 4. The Stem. _ 
163. Anatomically the stem consists of a cellular and a fibro-vascular system. 
The cellular system developes both vertically, as stems increase in length, and hori- 
zontally, as they increase in diameter. The fibro-vascular system is gradually intro- 
duced vertically, and serves to bind together and strengthen the cellular. It may 
be compared to the bony-skeleton, the cellular to the flesh. 
164. The stems of flowering plants are formed on one or other of the two follow- 
ae 
= Zt Exogenous structure, in which the woody system is deposited in annual 
concentric layers between a central pith and an exterior separable bark. 
Plants having this structure of stem are Exogens. 
the Endogenaus structure, in which the woody system is deposited, not in con- 
centric rings, but in separate, small bundles or threads of woody-fibre, 
running through the cellular system without apparent order. In such 
stems there is no distinct pith, nor separable bark. Plants having this 
structure of stem are called Endogens. 
165. The stems of the lower cryptogamia consist wholly of cellular-tissue ; those 
of ferns have an imperfect fibro-vascular system derived from the bases of old petioles. 
166. Tar am @eapenodie shean a aee-layticGr riage weed aunacelly ened waleall : 
the outermost preceding layer and the inner surface of the bark. In an endogenous, 
the new wood bundles are mingled with the old, or deposited toward the centre of 
the stem, from which they are gradually pressed out toward the circumfe as 
new wood continues to be deposited within them. In an Exogen therefore the oldest, 
hardest, and most compact wood is found toward the centre of the stem ; in an En- 
dogen towards the aeremntere P igor Fi wei naar? Sea 
167. Anatomists distinguish the follow l sf Fae sine growing rica “gars 
siste of cellular tissue, occupying the centre or longitudinal axis of the 
®) the medullary-sheath, which surrounds and encases the pith, abounds in 
vessels (153, c), and is in direct connection, through its ramifica- 
tions, with the leaf-buds, and the veins and nerves of leaves. 
{c) the wood, which lies directly on the medullary-sheath, is formed of woody- 
tissue (1538, b), through which, in most cases, ducts (153, ¢), variously 
disposed, are in A new circle of wood is annually formed, on 
the outside of the circle of the previous year ; the age of a stem therefore 
may be ascertained, in a large number of cases, by counting the num-~ 
bers of its rings of wood: in some cases of tropical trees and ever-green 
trees of temperate climates, several rings of wood are formed in a year. 
The older and denser, comparatively sapless wood, is called heart-wood 
or duramen, and is often coloured ; the younger, living and incompletely 
formed is the sapwood or alburnum, and is usually white. as 
