THE LEAVES OF VASCULAR PLANTS. Q33 



seen swollen into kinds of oblong tubercules along their 

 ramifications. 



Care must be taken not to confound with the true 

 nerves, certain lines produced upon some leaves in their 

 infancy by the impression of the median nerve, or the 

 border of other leaves. This is observed in a singular 

 manner in Ocotea (PL 10, fig. 1), where the leaf, inde- 

 pendently of its ordinai'y nerves, presents an oblique 

 line. The straightness, obliquity, and variety of posi- 

 tion of this line, are circumstances which clearly distin- 

 guish it from true nerves. 



When the fibres spread out to form the limb of the 

 leaf, they can do it in two difierent manners (whether it 

 takes place at the extremity of the petiole, or at their 

 issuing out of the stem) ; they may either spread out 

 upon the same plane, as is most frequently the case — this 

 forms the common flat leaves ; or they may diverge in 

 all directions, forming the cylindrical, swollen, or tri- 

 angular leaves of certain succulent plants. This last 

 disposition of the nerves can so easily be referred to the 

 di\'ision of flat leaves, that it will be sufficient for us to 

 explain these latter in detail. 



The limb of a fiat leaf presents three distinct parts: 

 — 1st, the upper surface ; — 2d, the lower one ; — 8d, the 

 intermediate space which, by analogy with Carpological 

 language, I shall call the Mesophyllum. "We will first 

 examine this last organ, which constitutes the body of 

 the leaf. 



The mesophyllum is formed of all the ramifications of the 

 nerves, and of the cellular tissue which fills up the inter- 

 vals between them and surrounds them : the less these 

 ramifications separate from the same plane, the thinner 

 is the leaf; on the contrary, the more they separate from 

 this plane, the thicker will it be, and the more must the 

 cellular tissue be developed to fill up the intervals. 



