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 fi-equent on 

 plants iidiabiting dry or exi^osod 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 cliiefly of carbonic acid and nitrogen, or nitrogenous 

 compounds, dissolved in water. 



162. rarasites are plants whose roots bury themselves in the cellular 

 structiu'c of other plants, and subsist on nourishment sucked out of the 

 plant which they attack. EpipJiytes 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 fibre- 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 

 akjiual eoncentric layers between a central pith ?indi an exterior 

 scp<(rable bark. Plants having this structure of stem are 

 EKOgens. 



the Ei/doge)ioHs structure., in which the woody sj^stem is deposited, 

 not in concentric rings, but ia 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 separal)le bark. Plants having this structure of 

 stem are called Sndogens. 



165. The stems of the lower Cr^-ptogamia consist wholly of cellular 

 tissue ; those of Ferns have an imperfect fibro-vascidar 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 covu'se. In an Exogen 

 therefore the oldest, hardest, and most compact wood is foimd 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 



