THE LEAVES OF VASCULAR PLANTS. 289 



by a kind of ring ; w^e see this in the floral leaves of 

 Seseli Hippomarathrum, for example. 



Leaves which are neither opposite nor verticillate 

 cannot, on account of their position, be naturally united 

 by their edges, but there happens another phenomenon 

 analogous to the preceding : — if they are sessile, and 

 their lower parts are sufficiently developed to allow of 

 their making the circuit of the stem, it happens in some 

 cases that the two edges are joined together at the base, 

 and the limb thus surrounds the stem, which seems to 

 pierce it. It is this which takes place in Bupleurum 

 perfoUatum. We say then that the leaf is Perfoliate, 

 a name which has also been extended to cases of oppo- 

 site or verticillate ones, joined together ; for example, 

 Crassula perfoliata, Triosteum perfoUatum, Sec. 



In all these different cases the annular limb, which 

 results from the union of several leaves or lobes of the 

 same leaf, may present two positions : — either it is 

 entirely spread open, and forms a ring which cuts the 

 stem at nearly a right angle ; or it is more or less erect, 

 and then following the direction of the stem, it sur- 

 rounds it by a kind of sheath, more or less prolonged. 

 In this last case it happens sometimes that the sheath 

 remains distinct from the stem, and does not adhere to 

 it, as in the Gramineae ; sometimes its internal surface 

 adheres to the stem, and seems to make part of it; it is 

 in this manner, for example, that the leaves oi Salicornia 

 sheathe the stem and adhere to it. 



In reference to this, some Ficoids present a \exy 

 singular appearance : their leaves are opposite, very 

 thick, and so joined together by their edges (connate) 

 that each pair contains within it the young shoot about 

 to be developed; when the top of the stem enlarges, 

 it breaks the union of the leaves, which serve as a bud 

 for it, and it appears beyond it, having in the same 

 VOL. I. u 



