6 



BULLETIN 223, T7. S. DEPARTMENT OP AGRICULTURE. 



top. The sides or lateral faces are flat or somewhat concave, and 

 their inner margins, at first slightly converging, as they approach the 

 top are drawn so near together as to give a triangular cross section 

 to the rib. 



The characteristic form or habit of tree tops of each date variety, 

 due to the curves of the leaf blades, is largely determined by the 



flexibility of the 

 ^f\ /\ rachis. This is gov- 



erned in part by the 

 degree of firmness and 

 elasticity of the fiber 

 of the rachis, but 

 more by the way in 

 which the diameter 

 diminishes along the 

 different faces. 



The feathery grace 

 of the Areshti variety 

 is due to the leaves 

 maintaining consider- 

 able rigidity in the 

 lower portion, but 

 with the rachis di- 

 minishing to a deli- 

 cate and slender flexi- 

 bility at the apex. 

 The broad, open top 

 and loose, lazy curves 

 of the Rhars leaves are 

 due to a rather rapid 

 diminishing in the di- 

 ameter of the rachis a 

 short distance above 

 the base, yet main- 

 taining too much size 

 in the apical portion 

 to give grace or airi- 

 ness. In the Thoory 

 and other varieties of that class the thick, strong rachis holds its size 

 and rigidity well up in the blade, curving only when forced out by 

 the growth of the inner leaves. Such trees have a stiff and uncom- 

 promising aspect through the entire top. 



The relative size and form of the rachis in different portions of the 

 blade are so characteristic in the different varieties and such impor- 



Fig. 2. — Diagram showing how the size of the date leaf is affected by 

 the angles which the pinnae form with the rachis. A represents the 

 leaf as lying flat, the dotted lines (6 6) showing the breadth the leaf 

 would have with the pinnae at 90° of axial divergence, the solid lines 

 (c c) showing the breadth of the leaf with pinnae diverging 45° from 

 the axis. B represents a cross-section view of the leaf, & showing the 

 pinnae lying in the blade plane, or flat, but with an axial divergence 

 of 45°; c shows the same pinnae also diverging 45° from the blade 

 plane. 



