DIV. I 



MORPHOLOGY 



111 



parallel venation the parallel main nerves are usually united by weaker cross 

 veins. Netted or reticulately-veined leaves in which the side veins run from 

 the median main nerve or MIDRIB are further distinguished as PINNATELY VEINED 

 (Fig. 126), or as PALMATELY VEINED (Fig. 122, 135 sb), when several equally strong 

 ribs separate at the base of the leaf-blade, and give rise in turn to a network of 

 weaker veins. Parallel venation is characteristic, in general, of the Mono- 

 cotyledons ; reticulate venation, of Dicotyledons and of some Ferns. 



Internal Structure. In structure foliage leaves exhibit consider- 

 able variety but are usually markedly dorsiventral (bifacial), the tissues 

 towards the upper side being different from those below (Figs. 127, 

 130). 



Many leaves, however, are similarly constructed above and below (equifacial, 

 centric, Figs. 187, 193). This is the case especially in forms which grow in 

 relatively dry situations, exposed to strong sunlight, but also occurs in submerged 

 aquatic plants. 



(a) NERVES. Within the nerves or veins one or more vascular 

 bundles run. The abundant branching of these bundles to form a 

 fine network is very characteristic of 

 the leaf-blade and is shown clearly in 

 leaf skeletons obtained by macerating 

 leaves. 



The structure of the vascular bundles 

 in the lamina corresponds on the whole 

 to that seen in the stem. In Phanero- 

 gams the bundles are usually collateral, 

 and since they are continuations of the 

 leaf-trace bundles from the stem the 

 xylem is directed towards the upper, 

 and the phloem towards the lower 

 surface of the leaf. 



The xylem parenchyma of the bundles in 

 the leaf usually forms flat plates, which in 

 cross-section appear as radial rows of cells in 

 the vascular tissues. 



As the bundles continue to ramify in the 

 leaf-blade they become smaller and simpler in 

 structure. The vessels first disappear, and only FlG 128 . _ Termination of a vascular 

 spirally and reticulately thickened tracheides bundle in a leaf of Impatiens parvi- 

 remain to provide for the water conduction. flora, (x 240. After SCHENCK.) 

 The phloem elements undergo a similar reduc- 

 tion. In Angiosperms, in which the sieve-tubes are accompanied by companion 

 cells, the sieve-tubes become narrower, whilst the companion cells retain their 

 original dimensions. Finally, in the cells forming the continuation of the sieve- 

 tubes, the longitudinal division into sieve-tubes and companion cells does not take 

 place, and TRANSITION CELLS are formed. With these the phloem terminates, 

 although the vascular portion of the bundles still continues to be represented 

 by short spiral tracheides. The ultimate branches of the bundles terminate blindly 

 (Fig. 128). 



