VEGETABLE ORGANOGRAPHY. 



In the limb, whether sessile or petiolated, are distin- 

 guished at first the Nerves ; that is to say, the bundles 

 of fibres which separate from one another at the base of 

 the limb, and form as it were the skeleton. The first 

 bundles which spring from the base of the limb, or the 

 prolongation of the petiole, bear the name of Primary 

 Nerves ; their immediate ramifications are called Se- 

 condary Nerves ; the divisions of these are the Ter- 

 tiary Nerves ; and several orders of them might be 

 recognised, until we arrive at the last ramifications of 

 the fibrous bundles in which the vessels are found iso- 

 lated. All these ramifications collectively foi'm the 

 fibrous tissue, which is as it were the skeleton of the 

 leaf. 



The interval between the nerves — the primary, se- 

 condary, &c. is more or less filled up by the develop- 

 ment of cellular tissue ; and it is this, strictly speaking, 

 which forms the Parenchyma of the leaf: but it must 

 be remarked, in order to comprehend the customary 

 sense of terms employed in Botany — 1st, that the nerves 

 which project but little or not all, but yet are visible, 

 are called Veins ; and 2d, that under the name of 

 Parenchyma is generally confounded not only the cellu- 

 lar tissue, properly so called, but also the last ramifica- 

 tions of the fibrous tissue, or the scarcely apparent 

 veins. 



The nerves of leaves differ much in their thickness, 

 sometimes being very considerable, at others scarcely or 

 not at all projecting beyond the parenchyma. In general 

 they go on regularly diminishing in thickness from the 

 base of the limb to their extremities. I do not know 

 that this law has more than a very small number of 

 exceptions : the most remarkable is the leaf of an 

 unknown tree from Cayenne, of which 1 possess 

 branches in my Herbarium, and where the nerves are 



