3o6 PRIMARr ARRANGEMENT OF TISSUES. 



longitudinal lamellce of the thick middle layer of the leaves of Typha and Sparga- 

 nium, which are separated by large air-cavities, there lie 1-3 longitudinal bundles : 

 numerous smaller ones are placed at the outer limit, abutting on a bundle of hard 

 hypodermal sclerenchyma. In the thick leaves of the IMesembryanthema (M. linguse- 

 forme and its allies, M. barbatum, imbricatum, stramineum, &c.) the main branch- 

 bundles run longitudinally through the centre of the middle layer, and send out 

 branches on all sides in an obliquely apical direction, which extend with their reti- 

 culately connected ultimate ramifications to its outer limit. Also in the thick 

 leaves of the Crassulacea;, and even of the Semperviva, divergences of the bundle- 

 branches and meshes are found towards the surfaces : this is most strikingly seen in 

 the branch-bundles which end at the surfaces in the thick-leaved species of Crassula 

 to be described below (Sect. iii). 



On the arrangement of the bundles peculiar to the fructiferous leaves of Platy- 

 cerium, which, from its form, belongs to this category, and upon the peculiarities of 

 vascular bundles in the sporangium-bearing leaves of Ferns generally, which will not 

 be further treated in this work, the Pteridographic literature should be referred to, 

 especially Mettenius, Filices horti Lipsiensis. 



It is well known that within all types there is found the greatest variety in the direc- 

 tion of the bundles, and of the corresponding ribs of different orders, in their divergence, 

 number, and relative strength. It is the province of the special description of plants to 

 enter into the details of these conditions of nervation. 



The examples of amphibious plants, of leaf-like phyllode branches, of plants with 

 rudimentary leaves and cortical networks of bundles, show that the bundle-system may 

 be immediately dependent upon different adaptations. Sometimes it is altered accord- 

 ing to different adaptations of morphologically equivalent members: the submerged 

 leaves of the water Ranunculi and of Elatine Aisinastrum belong to the first type with 

 a separate course of the bundles, while the aerial leaves of the same species belong to the 

 reticulate type : sometimes a similar bundle-system appears in morphologically different 

 members subject to similar adaptation : phylloclades of INIyrsiphyllum, and Ruscus, as 

 compared with foliage leaves of allied plants. 



On the other hand, the different main and subsidiary forms of the vascular system cannot 

 in most cases be referred directly to adaptative causes. Within a narrower or wider 

 circle of relationship the same type of nervation occurs, whether the adaptation be similar 

 or different, and the converse is also the case. Further the nervation is to a great extent 

 independent of the form of the leaf. After what has been said above it is superfluous to 

 adduce examples of this. 



Among the large divisions of the vegetable kingdom the Dicotyledons show the greatest 

 uniformity of ground-plan of nervation, since their aerial foliage leaves, with the one ex- 

 ception of the narrow-leaved Eryngia, all belong to the reticulate type ; and individually 

 they show the greatest variety, since in this type variations and combinations of dif- 

 ferent points of detail are possible, and really exist. 



Among the Monocotyledons the great majority of forms belong to the striated type, 

 which shows generally an extraordinary uniformity in the main phenomena. Only the 

 few above enumerated families and genera appear as remarkable exceptions, since 

 some of them correspond exactly to the reticulate type of Dicotyledons, while others 

 approach it. 



Among the Gymnosperms Gnetum alone (in accordance with other points in its mor- 

 phology which approach nearest to the Dicotyledons) has a truly Dicotyledonous nerva- 

 tion : the pinnae of Stangeria have only single marginal anastomoses : all other forms have 

 bundles with separate course. 



