BUNDLE-SYSTEM IN THE LEAVES. 305 



3. Both internal and peripheral ends occur in all (?) Ferns of this category, though 

 in many (e.g. Ophioglossum vulgatum, pedunculosum, Platycerium) the peripheral ends 

 are but few; further in numerous Dicotyledons, and species of Smilax and Dioscorea. 

 In the flat leaves of the Dicotyledons the terminating bundles sometimes run along 

 strong (marginally directed) nerves, sometimes they are small short branches which 

 terminate at the margin, and especially in the marginal teeth. It is a phenomenon 

 of especially frequent occurrence that two or more branches, one from each side, unite 

 near to the margin with one stronger, marginally-directed bundle ; they then run 

 together outwards to the margin, so that in other w^ords the free ends pass off from 

 the sympodial marginal bundle. This is the case in the leaves of Primula sinensis, 

 Papaver, Brassica, Fuchsia, Calendula, Cucurbita, Mercurialis, and Camellia japonica. 

 The leaves of the CupuUferse^, Betulaceae, Myrica, Planera, Ulmus, species of Tri- 

 folium, Tropseolum, &c. are further examples belonging to the present category, 

 which however have not been exactly investigated as regards the last-mentioned 

 condition. Small branches go from the sympodial marginal branches towards the 

 margin, in the investigated species of Smilax and Dioscorea. 



Sect. 92. As regards the arrangement of the bundles as seen in a transverse 

 section of the foliar expansion, they run, of course with the exception of their peripheral 

 ends, within the other tissues, not superficially, those in the ribs being enclosed by the 

 collenchyma and sclerenchyma, or by the aqueous tissue (p. 116), which forms the 

 mass of the projecting rib, or rarely by the chlorophyll-parenchyma, which extends in 

 greater or less bulk into the rib. For those which do not lie in prominent ribs, 

 that is, for the smaller branches of most ramifying bundles, and for all the bundles 

 of many fleshy Monocotyledonous leaves, the rule holds, that they lie close within, 

 or below the inner limit of the chlorophyll-containing palisade-cells or rows of cells, 

 which are perpendicular to the surface of the leaf, but are not embedded in that 

 tissue. Thus in the bifacial leaves (comp. Chap. IX) they lie in the spongy paren- 

 chyma, where this borders on the palisade layer ; in leaves whose tissues are arranged 

 on the centric type, at the periphery of the (colourless) middle layer ; in interme- 

 diate forms, such as Dianthus Caryophyllus and Crassulacege, at the point where the 

 rows of cells which run inwards perpendicularly from the whole leaf-surface meet at 

 the middle of the leaf. In the first and last cases the bundle-system is accordingly 

 extended, in reference to a horizontal leaf, in a horizontal plane, in the second case 

 on the surface of a much-flattened hollow body. 



I know of no exceptions to this rule in bifacial leaves. In concentrically constructed 

 expansions with a relatively thin middle layer, consisting only of a few layers of cells, 

 all the bundles are often enclosed within the latter (e. g. leaves of Statice monopetala, 

 Phyllodes of Acacia marginata), or the stronger bundles are within the middle layer, 

 and only the thinner branches at its outer limit (Flakea ceratophylla, Acacia longi- 

 folia, Huegelii). In the leaf of Agave americana the thick middle layer is traversed 

 by several series of bundles, which, with the exception of the central pnes, stop short 

 towards the margin ; they run parallel to the surface of the leaf, and are connected by 

 anastomoses : besides these there is an external series which extends round the whole 

 leaf, at the limit between the chlorophyll layer and the middle layer. In each of the 



1 Von Ettingshausen, Blattsk. d. Dicotylen, Taf. I, II, &c. 



y 



