SECT. I 



MORrnOLOGY 



39 



each separate leaf is disposed 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 AESTIVATION. 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. 40 h). If, as frequently 

 occurs in flower-buds, the margins of the floral leaves successively 

 overlap each other in one direction, 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 



Fig. 40. — Transverse section of a bud of I'opulus 

 nigra, k, Bud-scales showing imbricated 

 aestivation [vernation] ; I, foliage leaves 

 with involute vernation [ptyxis] ; s, each leaf 

 has two stipules, (x 15.) 



Firi. 41.— Transverse section of a leaf- bud 

 of Taufja canadensis, just below the 

 apex of the shoot, showing a j\ diverg- 

 ence. (X circa 20, after Hofmeister.) 



most easily recognised in cross-sections of buds (Fig. 41), particularly 

 in sections showing the apex of the vegetative cone (Figs. 31, 33E). 

 From such sections it is evident that the developing leaf-rudiments 

 stand in the relative position to the pre-existing leaves which 

 best utilises the available space. According to Schwendener (^^) 

 the arrangement of the leaves on the axis is determined by purely 

 mechanical causes ; it is dependent on the pressures and tensions 

 induced by the leaves which always stand in contact at their origin. 

 The original arrangement of the young leaves may be modified as 

 growth continues owing to the mutual pressure they exert. If the 

 axis does not grow in length, but only in thickness, as the rudi- 

 mentary leaves increase in size, their points of insertion will be dis- 

 placed laterally by longitudinal pressure ; if the axis increases in length, 

 and not in thickness, the insertion of the leaves will be displaced by 

 a transverse pressure. The arrangement of the leaves would also be 



