3,04 PRIMARY ARRANGEMENT OF TISSUES. 



Dioscoreae belong to this type^and also many Smilacese, especially Smilax, Taccaceae, 

 Lapageria, Philesia, &c.; among the Gymnosperms, Gnetum; finally, the reticulate 

 Fern-leaves, which represent in Pteridography the types of nervation of Gonio- 

 phlebium, Phlebodium, Doodya,' Marginaria, &c. The typical Aroideae, the broad- 

 leaved Potamogetons, and Hydrocharis belong also to this category, but appear 

 as intermediate forms between the type under consideration and the striated type. 



It is well known that the bundles of this type enter the expansion singly or several 

 together in a midrib, which runs towards the apex, and give oif from it branches of 

 the first order, arranged in a pinnate manner, into the two halves of the leaf (Folia 

 penninervia) ; or several separate main bundles diverge from the insertion of the leaf, 

 and are also in their turn pinnately branched (Folia palmatinervia, peltinervia, tripli- 

 nervia, &c.). The branches of higher orders are also sometimes arranged in a 

 pinnate manner, sometimes they show (true or false ?) dichotomy. The number of 

 orders of branching usually ranges in the Phanerogams from five to eight. In the 

 Ferns the branching is simpler in all respects than in the Phanerogams of this series. 

 It has already been noted above that branches of each order may end free or 

 anastomose peripherally or internally. As regards the occurrence of these modes of 

 ending the following cases occur : — 



I. In rare instances reticulate connection between all branches, free ends 

 occurring at most only in the apex of the leaf. In many succulent plants — species 

 of Sempervivum, Mesembryanthemum — all free endings are absent, or at least have 

 not hitherto been proved to exist : but an exact investigation of this point is still to 

 be desired. Bijt by no means all succulent plants belong to this type : Salicornia, 

 e. g. has numerous internal endings in its cortical reticulum of bundles (p. 297), and 

 the shrubby species of Crassula haye numerous peripheral ones. Among Monocoty- 

 ledons those Aroids which have been investigated (species of Anthurium, Pothos, and 

 Monstera, Calla, Richardia) belong to this series, and further Hydrocharis and Pota- 

 mogetqn. - All' these have a free end at the apex of the leaf. In the two last-named 

 genera the main bundles have a course similar to that of the longitudinal bundles of 

 the striated type, the transverse branches which connect them are repeatedly branched, 

 and the branches are connected into a net with angular meshes. In the Aroideae 

 also the course of the main bundles reminds us of that of the striated Monocotyle- 

 donous leaves. Between them branches of many orders form a complex angular 

 network. Free internal eiids are absent, or present only rarely here and there. 



2. Free internal endings within the meshes, and no free peripheral endings. Close 

 to the margin of the flat leaf runs the sympodial marginal bundle,' which limits all 

 the marginal meshes externally, and gives off no branches in a peripheral direction. 

 The leaves of species of Ficus and Banksia, of Cocculus laurifolius, Buxus, Quercus 

 pedunculata', and Psoralea (Fig. 146), maybe named as examples of this pheno- 

 menon ; apparently very many leaves, especially tough long-lived ones, with an entire 

 margin, resemble these. Still I must cite no further examples, since the existing 

 works OH the coarser nervation do not permit of a certain decision whether short 

 and thin free-ending bundles pass peripherally from the sympodial marginal bundle 

 or not. 



' Frank, Botan. Zeitg. 1864,' p. 380. 



