264 DR J. M. MACFARLANE ON THE 



Leaf. — (b) Petiole one-third beneath insertion of leaflets. 



In C. purpureus the petiole at this region is greatly flattened out, so as to be 

 crescent-shaped. Three to four layers of the cortex cells beneath the epidermis contain 

 chloroplasts ; the sclerenchyma is principally produced as a crescentic band beneath the 

 main bundle, and consists of one to two layers. Above the bundle it is feebly developed. 

 The phloem is slightly concave, but the main mass of xylem is nearly flat. Two lateral 

 bundles, which in position might be said to form the horns of the crescentic main bundle 

 mass, repeat in miniature the arrangement of it. 



In C. Laburnum the outline of the petiole is circular, except that it is traversed on 

 its upper face by a deep groove. Beneath the epidermis is a collenchyma layer and 

 internal to it are one to two layers with chloroplasts, succeeded by four to five layers of 

 pale large cells. The sclerenchyma and bundle masses are arranged as in the petiole 

 base, but the two lateral bundles which have been given off from the cylinder lie superior 

 to the sides of the cylinder, and repeat the arrangement of the deeply concave bundle in 

 miniature. 



In C. Adami the outline of the petiole is most nearly as in C. Laburnum, though it 

 is slightly flattened above and has only a shallow groove. A broken zone of collenchyma 

 cells lies beneath the epidermis, and is succeeded by two or three layers of cells with 

 chloroplasts, while three to four subjacent layers are large-celled. The sclerenchyma and 

 bundle masses resemble those of C. Laburnum, though the elements are smaller and more 

 feebly thickened. 



Altogether the entire petiole of C. Adami is very pronounced in its leaning towards 

 C. Laburnum, particularly in the massing of tissue elements, so much so that one might 

 on cursory examination conclude that the resemblance is complete. But many minor 

 points throughout prove the modifying action which C. purpureus has had ; thus the 

 difference in naked-eye outline, the incompleteness of the collenchyma layer, the reduced 

 width of the large-celled layers, and the smaller size of the elements, are all to be 

 explained as modified by it. 



Microscopically examined, the upper leaf epidermis of C. purpureus is almost identical 

 with the lower (Plate VIII. figs. 4, 5) as to cell shape, number of stomata in a given area, 

 and absence of hairs, though a very few of the last may be detected along the ribs. 



In C. Adami numerous stomata occur on the upper surface (Plate VIII. figs. 6, 7), 

 though less abundantly than on the lower, being in the proportion of 4 : 5. In C. Labur- 

 num the stomata are entirely confined to the lower surface, if we except a narrow patch 

 on the upper epidermis, towards the base of the midrib and on either side of it. The 

 hairs which spring from the lower epidermis (Plate VIII. fig. 9) are short, blunt, 

 spindle-shaped, and minutely tuberculate, their presence giving a silvery sheen to the 

 surface. I regard it as a point in favour of its exceptional origin that the "Adami" 

 leaves should not have these in tolerable abundance. 



As regards relative distribution the stomata are as follows, under field of Zeiss' D 

 with No. 2 eyepiece : — 



