8 



BULLETIN 223, U. S. DEPARTMENT OF AGRICULTURE. 



ward (decurrent) along the lateral face for an inch or two. With the 

 fold of the blade slightly broader than the face of the rib, a free wing 

 results, and a series of these with corresponding wings from the oppo- 

 site side of the rachis may form a narrow channel along the middle 

 of the inner face of the leaf. When such wings and channels are 

 conspicuous they constitute good varietal characters and also are 

 of some importance in affording harbors for scale insects, particularly 

 Phoenicococcus marlatti. 



When the new leaves issue from the center of the crown, three, 

 making a complete circle of the stem, usually follow in close succession 

 and are crowded into the form of an irregular cylinder. This gives to 

 each emerging leaf the form of a third of a cylinder with an acumi- 

 nate apex, the pinnae being folded as compactly as the ribs of a fan 



upward against the rachis. With 

 the expansion of the rachis the 

 pinnae diverge and the cushiony 

 pulvinus at the base of each, at 

 first scarcely noticeable, rapidly 

 expands, pushing the pinna to its 

 characteristic angle and holding 

 it there securely. 



ANGLES OF PINNAE WITH THE RACHIS. 



The difference in appearance of 

 the leaves of varieties of dates is 

 largely due to the different angles 

 at which the pinnae diverge from 

 the rachis. At first sight these 

 are confusing and are best under- 

 stood by constructing two imag- 

 inary planes parallel with the leaf and vertical to each other. (See 

 fig. 3.) Note (1) the plane of the blade (PB), which would divide the 

 leaf into an inner or ventral half and an outer or dorsal half, and (2) 

 the plane of the rachis (PR) at a right angle to the first and dividing 

 the leaf into right and left halves. 



If all pinnae lay in the first plane there would be only two ranks, a 

 right and a left rank, and the blade would be broad and flat, its out- 

 lines determined by the length and axial angles of the pinnae. If the 

 pinnae were inserted vertically or at 90° to the second plane, they 

 would project from either side like the teeth of a double comb. 



Actually but few pinnae in Phoenix dactylifera lie in either position, 

 but the exact position of any pinna may be recorded with precision 

 by determining its angle with each of these two planes. 



Fig. 3. — Diagram showing the construction of imag- 

 inary planes bisecting the date leaf longitudinally, 

 by means of which the angles of divergence of the 

 pinnsefrom the rachis are read: PR, Plane of the 

 axis, which would divide the leaf into right and 

 left halves; PB, plane of the blade, dividing the 

 leaf into dorsal and ventral halves. 



