J^i<2 VEGETABLE ORGANOGRAPHY. 



than ordinarily, and perform in some respects the 

 physiological function of leaves. It is possible that to 

 this class of phenomena the singular structure of Cycla- 

 me7i linearifolium ought to be referred. Most bracts and 

 several scales of buds are degenerations of leaves analo- 

 gous to those which I have been pointing out. If the 

 scales or rudiments of leaves of Monotropa be compared 

 with the base of the petioles of Pyrola, we shall consider 

 them as petiolary sheaths, and, by analogy, the same 

 ought to be admitted with respect to Orohanche, 

 Lathroca, &c. 



The petiole of the Polygoneas is also furnished at its 

 base with a membranous and sheathing appendage, 

 which is called the Ochrea ; but its anatomical history 

 still presents some obscurity, and it also may almost be 

 considered as a petiolary sheath, or as formed by intra- 

 axillary stipules united together. 



3d. It sometimes happens, especially when the limb 

 is not developed, that the petiole, without being sheath- 

 ing at its base, dilates throughout its entire length, 

 assuming a state intermediate between foliaceous and 

 petiolary, and then it receives the name of Phyllo- 

 DiUM ; thus, when most of the Acacias of New Holland 

 are examined, we see that in their infancy they present 

 bi-pinnate leaves, with a slender, nearly cylindrical 

 petiole. As the plant advances in age, the number of 

 leaflets diminishes, the petiole dilates, and by degrees 

 the leaflets completely disappear, and all the leaves are 

 reduced to petioles dilated into Phyllodia. These are 

 flat, coriaceous, firm, always entire at their edges, provided 

 with longitudinal nerves, which are the traces of the fibres 

 of which the petiole is composed, and constantly placed 

 upon the stem in a contrary direction to true leaves; 

 that is to say, their plane is nearly vertical instead of 

 being horizontal y or, in other terms, their surfaces are 



