418 



RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. 



nodes. The torsion of the leaf-stalks of the Judas Tree (Cercis SiliqvMstrwm), 

 where this alone occurs, i.e. without a simultaneous twisting of the internodes, is 

 particularly striking. The leaves of the plant, as can be seen on the erect twigs, 

 and especially well in the suckers, are arranged in the one-half phyllotaxis, i.e. 

 in two rows. The leaf -blades on the erect branches are parallel with the ground. 

 If a sucker be cut off and held horizontally, all the leaf-laminae will be directed 

 at right angles to the earth. One might perhaps expect that they would also 

 assume this direction if the twig had grown horizontally. Anything Ijut that, 

 however. The stalks of all the leaves twist round instead, until the laminae, 

 or blades they bear, are again placed in a direction parallel with the gi-ound on 

 the horizontal branches, and the result is that the leaves on all the branches of 



Fig. 108. — Horizontally growing leafy twig of the Paper Mulberry-tree (Broussotietia papyri/era). 



the Judas Tree, whether erect, oblique, horizontal, or inclined towards the earth, 

 always present the same attitude to the incident light. 



The third case, the alteration in the inclination of the blade to the leaf -stalk, 

 which, on the whole, is but seldom met with, is represented in the best known 

 example, the cursorily mentioned Japanese Paper Mulberry {Broussonetia papy- 

 ri/era) in fig. 108. In this plant the leaves are decussate, i.e. arranged in four 

 rows, each pair of leaves being inserted at the same level, and the successive 

 pairs alternating at right angles. In erect twigs, therefore, they display the 

 arrangement seen in the twigs of maple (see fig. 106) or of Diervilla (see 

 fig. 107 ^). The following alteration, however, is seen to occur in the horizontal 

 branches of the lower boughs of the Paper Mulberry. In each pair of leaves the 

 leaf -stalk of one leaf becomes parallel to the surface of the ground, and lies in the 

 plane of the blade it supports, which is also almost horizontally extended, or but 

 slightly inclined. The other leaf -stalk, however, rises vertically from the horizontal 

 twig; the leaf-blade it supports is bent down at right angles from it, and conse- 

 quently is here again parallel to the surface of the gi-ound. A slight torsion of 

 the internodes, a shortening of the erect leaf-stalks, and a diminution of tlie leaves 



