44 



THE LEAF BCD. 



plant in the internodes of the stem or branches, on the root, or even 

 leaves. 



208. Causes and examples. Such buds generally result from some 

 abnormal condition of the plant, from pruning or other destruction of 

 branches or stem above, while the roots remain in full vigor ; thus de- 

 stroying the equilibrium of vital force between the upper and lower 

 axis. The leaf of the walking-fern emits rootlets and buds at its apex ; 

 the leaf of Bryophyllum from its margin, each bud here also preceded by 

 a rootlet. Some plants are thus artificially propagated in conservato- 

 ries from the influence of heat and moisture on a leaf or the fragment 

 of a leaf. 



209. Vernation or pr.efoliation are terms denoting the mode oi 

 arrangement and folding of the leaf-organs composing the bud. This 

 arrangement is definitely varied in different orders of plants, furnishing 

 useful distinctions in systematic botany. 



210. The vernation of the bud is exhibited in an interesting man- 

 ner bv making with a keen instrument a cross-section of it in its swollen 

 state, just before expansion ; or it may be well observed by removing 

 one by one the scales. 



211. The forms of vernation are entirely analogous to those of 

 aestivation, and denoted by similar terms. We shall here notice only 

 such as are more peculiar to the leaf-buds. 



212. Vernation is considered in two different aspects, first, the 

 manner in which the leaf itself is folded ; second, the arrangement oi 

 the leaves in respect to each other. This depends much upon the 

 phyllotaxy. (§ 220.) 



71 



72 



73 



74 



75 



7C 



Vernation, 71, of oak leaf ; 72, of Liriodcndron (tulip tree) ; 73, of fern ; 74, of carex ; 

 75, sage ; 76, iris. 



213. Each leaf alone considered is either flat and open, as in the 

 mistletoe, or it is folded or rolled, as follows : 



Reclined, when folded crosswise with apex bent over forward towards 

 the base as in the tulip-tree. 



Conduplicate, when folded perpendicularly, with the lateral halve* 

 brought together, face to face, as in the oak. 



