MORPHOLOGY 



Prickles vary considerably in number, they are not arranged in any 

 definite manner, and in some cases are entirely absent. 



Vernation and Estivation. — A section through a winter bud 

 shows a wonderful adaptation of the rudimentary leaves to the narrow 

 space in which they are confined (Fig. 38). They may be so disposed 

 that the separate leaves- are spread out flat, but more frequently 

 they are folded, either cross-wise or length-wise on the midrib 

 (conduplicate), or in longitudinal plaits, like a fan (plaited, plicate) ; 

 or they may be crumpled with no definite arrangement of the folds ; 

 or each leaf may be rolled, either from the tip downwards (circinate) or 

 longitudinally, from one margin to the other (convolute), or from both 

 margins towards the midrib, either outwards (revolute) or inwards (in- 

 volute, Fig. 38, 1). The manner in which each separate leaf is disposed 



Fig. 38. — Transverse section of a bud of Popvlus 

 nigra, k, Bud -scales showing imbricated 

 sestivation ; I, foliage leaves with involute 

 vernation ; s, each leaf has two stipules. 

 (X15.) 



Fig. 39.— Transverse section of a leaf-bud 

 of Tsuga canadensis, just below the 

 apex of the shoot, showing a f s diver- 

 gence. (After HOFMEISTER.) 



in the bud is termed VERNATION. On the other hand, the arrangement 

 of the leaves in the bud with respect to one another is designated 

 ^Estivation. In this respect the leaves are distinguished as free when 

 they do not touch, or valvate when merely touching, or imbricated, 

 in which case some of the leaves are overlapped by others (Fig. 38, k). 

 If, as frequently occurs in flower-buds, the margins of the floral 

 leaves successively overlap each other in one direction, obliquely or 

 otherwise, the aestivation is said to be contorted. 



The Arrangement of Leaves. — In all erect elongated shoots, and 

 still more so in dwarf shoots, it is apparent that there is a marked 

 regularity in the arrangement of leaves. This regularity may be most 

 easily recognised in cross-sections of buds (Pig. 39), particularly in 

 sections showing the apex of the vegetative cone (Fig. 29). From 

 such an apical section it is easily seen that the regularity in the 



