MORPHOLOGY 



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 

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

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

 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 ( 21 ) 

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

 mechanical causes ; it is dependent on the pressures and tensions 



FIG. 40. Transverse section of a bud of 



a lit ni. k, Bml-scales showing imbricated 

 ifstivation [vernation] ; I, foliage leaves with 

 involute vernation fptyxis] ; a, each leaf has 

 two stipules, (x 15.) 



Fio. 41. Transverse section of a leaf-bud 

 of Tsugii <<< iiiidensis, just below the 

 apex of the shoot, showing a -f 3 diver- 

 gence. (After HoKMKISTER.) 



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 

 rudimentary leaves increase in size, their points of insertion will be 

 displaced 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 affected by any increase or decrease in the size of the 

 vegetative cone, unaccompanied by a corresponding increase or 

 cessation of the growth of the rudimentary leaves. 



In developing flowers sudden changes in the relative position of 

 the parts often occur in which the apical cone undergoes rapid 

 increase in size while the leaf rudiments become smaller. Changes 

 in the usual position of the leaves may also be occasioned by the 



