BVNDLE-SYSTEM IN THE LEAVES. 



303, 



above described (p. 265) in the halms of many Monocotyledons have the same course as 

 in the leaves. 



It is only rarely that in leaves of Monocotyledons single transverse branches end 

 blindly in the surrounding tissue. It is more commonly the case for a longitudinal bundle 

 to arise as a branch from one of them. 



The huge leaf of Welivitschia is traversed longitudinally 

 by very numerous strong parallel bundles, these being con- 

 ne^cted in a ladder-like manner by transverse branches. 

 The transverse branches either run at right angles from 

 the longitudinal bundles, or obliquely, and sometimes directly 

 and simply from one longitudinal bundle to the other, some- 

 times converging and anastomosing one with another in the 

 narrow intervening spaces. Here and there a transverse 

 branch ends freely in the parenchyma, without reaching the 

 next longitudinal bundle ; and from each of the transverse 

 connections there often starts one short branch, which also 

 ends blind in the parenchyma, and is always directed to- 

 wards the base of the leaf (Fig. 145). The bundle-system is 

 accordingly similar in most points to that of the longitu- 

 dinally striated Monocotyledons, but the numerous internal 

 free ends correspond to the usual condition in the reticulate 

 Dicotyledons. — In the leaves of the above-named species of b 



Eryngium only transverse branches between the parallel longi- •'■c- i4s — weiwirscUa mirabiiis ; 



. J. 1 i_ jt J. :i • , 1 . •! t 1 piece of the network of vascular. 



tudmal bundles are round ; m other similar narrow-leaved bundles m the leaf, prepared free ; 

 species, as E. aquaticum, there are also free endings and reti- '^^''^f^^ ileart^tht tee'ofthe'^ 

 culate anastomoses. leaf- 



{b) In the reticulate type (Fig. 146) the bundles which enter the leaf undergo 

 branching of higher or lower order, and the branches are distributed over the whole 

 surface, run in different directions, and are sometimes 

 connected into polygonal or curved meshes, or some- 

 times end free, internally or peripherally. Meshes of 

 higher order are enclosed in those of lower orders. 

 The marginal sides of all marginal meshes form to- 

 gether in flat leaves a sympodial bundle, which follows 

 the margin : this is more or less near to the actual 

 margin, and is not uncommonly situated in the extreme 

 rnargin itself (e. g. Quercus pedunculata, Banksia, 

 Lauraceae, Cocculus laurifolius, and many other cori- 

 aceous leaves). The bundles which end free internally 

 arise as branches from the sides of the meshes, and 

 terminate in the area enclosed by these, often after 

 repeated short branching. 



As far as is known all foliar expansions of the 

 Dicotyledons, with the few exceptions above quoted, 

 belong to this type. The leaves of many land-plants, such as those of many species 

 of Trifolium, which, from the appearance of their coarse nervation, seem to belong 

 to the type with bundles pursuing a separate course, are still no exception; nor 

 are the small linear "one-nerved" foliage-leaves of Dicotyledonous plants, such 

 as species of Erica, Passerina, Fabiana imbricata. Among Monocotyledons the 



FIG. 146. — Psoralea bituminosa (40). The 

 last branchings of the bundles in a piece of 

 a leaflet ; at r the margin of the latter. 



