196 



PHYTOTOMY. 



. The leaves of those plants, whose stems contain scattered 

 and parallel bundles of spiral vessels and sap-vessels, with in- 

 tervening cellular texture, exhibit only parallel nerves, with- 

 out the appropriated veins, as in the Grasses, the Palms, 

 the Coronariae, the Irideae, and the Scitamineae. In Ferns 

 we observe a peculiar distribution of the nerves and veins. 

 These seldom anastomose with one another, but more com- 

 monly issue in clear and pellucid points, which, by a more 

 careful dissection, shew the extremities of the spiral vessels in 

 a vermicular form. Something similar is observed in the 

 Hypericum dubium of Smith, and in some species of Crassu^ 

 la. The distribution of the nerves and veins is carried pe- 

 culiarly far in the leaves of the Aroideae, and the Melasto- 

 meae, for they are united on the margin of the leaf by large 

 anastomoses, which run parallel with the margin. 



309. 



The nerves and veins of leaves are a continuation of the 

 bundles of spiral and sap vessels, and therefore they remain 

 uninjured during the maceration of the cellular texture. 

 They thus exhibit, frequently, a beautiful skeleton, the de- 

 licacy and almost endless ramifications of which are astonish- 

 ing ; (Seligman's Nahrungsgefasse in der Blattern der 

 Baume, Nurnberg, 1748, folio.) But the cellular texture, 

 which fills the interstices of this net-work, and which, on ac- 

 count of its juicy consistence, is distinguished by the name of 

 Parenchyma, is of as much importance as the net-work itself. 



The cells of leaves have a different structure, according as 

 they are nearer the upper or lower surface. Near the upper 

 surface, they are more extended in length, and take the form 

 of prisms or cylinders, in preference to any other shapes. 

 Besides, the upper surface of leaves is frequently covered by 

 an apparently impermeable, and somewhat brilliant epider- 

 mis, in which we observe no further remarkable organization. 

 On the lower surface, again, the cells are more extended in 

 width ; here and there also spaces void of juice occur, or 

 some of the cells seem to have lost their sap, and to have be- 

 come filled with air. Their partitions undergo a change, 



