INTRODUCTION TO BOTANY. XXV 
those of stems and leaves serve to protect the surface, or to control evapo- 
ration, and sometimes act also as absorbents. They are more frequent on 
plants inhabiting dry or exposed situations, or on very Alpine plants, 
which are alternately exposed, at brief intervals, to extremes of heat and 
cold. 
§ 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. Absorption 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 nitrogenous 
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. piphytes are those whose roots attach them- 
selves to other plants without penetrating into their cellular tissues. 
§ 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 horizontally, as they increase in diameter. The fibro- 
vascular system is gradually introduced 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 following types :— 
the Exogenous structure, in which the woody system is deposited in 
annual concentric layers between a central pith and an exterior 
separable bark. Plants haying this structure of stem are 
Exogens. 
the Endogenous structure, in which the woody system is deposited, 
not in concentric 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. In an exogenous stem a new layer or ring of wood is annually 
formed between 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, whence they diverge to- 
wards the circumference in the lower part of their course. In an Hxogen 
therefore the oldest, hardest, and most compact wood is found towards the 
centre of the stem ; in an Endogen towards the circumference. 
167. Anatomists distinguish the following parts in an Exogenous stem :— 
(a) the pith, which is only active in young stems or growing 
