240 MANUAL OF AGRICULTUI 





aa internally liard inseparable bark. These fascicles of fibres 

 run from the bark inwards and downwards towards the stem's 

 centre, effecting a firmly interlaced structure. 



The increase of an acrogenons stem takes place at its summit, 

 as exemplified in the case of ferns and tree-ferns. The whole 

 length is of nearly the same diameter, it is marked on the out- 

 side by the scars of leaves, whose bases, indeed, compose it. In 

 cross section there appears a cellular mass, often hollow in the 

 centre, with bundles of vessels interspersed throughout. Stems pro- 

 duce buds, or branches bearing them ; in some cases only at their 

 extremities, " terminal " buds, in whose destruction is involved 

 th3 death of the plant ; in other cases, both terminal and lateral. 

 Some buds, instead of developing into branches, become modi- 

 fied into thorns. Both branches and thorns have a continuity of 

 the central stern-substance, and are thus distinguished from such 

 prickles as, e.g., the briars, which are merely developments of the 

 epidermis, having no direct connection with the stem. The 

 functions of the stem are chiefly to support the leaves and flowers, 

 and to afford them a due exposure to the influence of sun and 

 air. The modifications of leaf form are endless. Microscopi- 

 cally examined, the leaf epidermis shows numerous hairs and 

 stomata. Immediately beneath it are discovered elongated or 

 ''palisaded" cells, having a close vertical arrangement; and 

 spaces are numerously interspersed, corresponding with the sto- 

 mata above. Inferiorly occur other cells more freely and openly 

 arranged, with the fibres and vessels constituting the veins of 

 the leaf running through them. The arrangement of leaf-veins — 

 the venation of leaves — affords another means of plant classifi- 

 cation. Along with exogenous stems, plants have a reticulated 

 venation : — with endogenous, a herring-bone, venation, or the veins 

 running parallel from the central vein to the leaf's margin. The 

 petiole or leaf-stalk attaches it to the stem. On any ordinary 

 tree it is more or less round and fibrous ; in the rhulDarb plant 

 again, it is thick and juicy, and constitutes the edible part. Some- 

 times it has almost the identical functions of the leaf itself ; in 

 sundry pines there is no distinction. The functions of the leaf 

 are occasionally assumed b}' the stipules — small leaflet-like bodies 

 at the base of the petiole, and very apparent in rose, pansy, and 

 clover plants. Frequently they form tendrils and sheaths of the 

 petiole. The midrib or central main vein of the leaf is a continu- 

 ation of the petiole. 



Leaves are either simple or compound, — the former when the 

 petiole carries but one blade, and has no joint above the point of • 

 union with the stem. Such are the leaves of the oak or beech 

 trees. Compound leaves have their blades subdivided into 

 separate distinct lengths, each of them being articulated to the 

 petiole, as in the case of the horse-chestnut. Leaf margins may 



