422 
Journal of Agricultural Research 
Vol. XXXI, No. 5 
water is transpired (pi. 1). This juxtaposition of the region of photo¬ 
synthesis with the region of cell formation and growth in the date 
palm has a very important relation to its general activity. 
“ Centralization of activities” is the keynote of date-palm growth. 
Its leaves, usually 8 to 12 feet long, are not, as is the case with many 
palms, loosened from the trunk with maturity, but the bases are 
strongly attached by tough fiber bundles which penetrate deeply 
into the cortex. From 10 or 12 to 20 leaves are pushed up from the 
center of the crown each year. These retain their efficiency four or 
five years, but gradually turn yellow and dry up, when they are 
pruned down to within 12 or 15 inches of the place of attachment by 
the date grower. 
The structure of the date-palm leaf is of the greatest importance 
in securing the insulation of the growth center. The rachis or midrib 
at the base of the blade, arbitrarily assumed to end at the lowest 
spines, may be 1 to 2 inches in diameter. From this point the lower 
portion, considered as the petiole, broadens and thickens more or less 
rapidly according to the variety and age of the plant. In mature 
leaves the base may be from 8 to 12 inches broad at the attachment 
to the trunk, and from 1 to 2 inches thick in the center, thinning out 
in the lower portion to the marginal thickness of the clasping sheath. 
Each leaf petiole has a triple-layered cylindrical sheath from 12 to 
20 inches long, at first white and crisp as lettuce leaves, but becoming 
a tough sheet of diagonally-crossed brown fibers upon expansion and 
exposure to the fight. The fine of attachment of the lower margin 
of the sheath to the trunk marks the beginning of the very short 
internode of the palm trunk. As the succession of these ribs and 
sheaths is developed from within, six or eight concentric layers may 
encircle the phyllophore and their combined strength binds the upper 
spirals of leaves firmly together 5 against the leverage of the winds on 
the broad leaf blades. These sheaths, with the persistent heavy 
fibrous bases, form a most efficient insulating and protecting layer 
surrounding the delicate meristematic tissue of the growing plant 
(pis. 2, 3, and 4). 
With the crowding up of new leaves in the center the sheath is 
finally torn from the sides of the base of the petiole, but the successive 
sheets, attached to the trunk by their lower margins, are tightly 
wedged behind the encircling bases, 13 nearly vertical ranks of which 
may be counted around the trunk. In the dry climate of Indio, 
Calif., and other date growing regions, these bases and sheaths usually 
persist for many years, covering the entire trunk with a thick mat 
of dry fiber interspersed with air spaces, and becoming a protective 
layer. 
Plate 1 gives an excellent idea of the external appearance of this 
protective layer; but Plate 3, a cross section of a date-palm trunk 
about 2 feet in diameter, shows very beautifully the imbricated 
arrangement of the spiral succession of leaf bases; while Plate 5, 
shows how effectively these layers insulate the tender tissues of the 
phyllophore. 
* It may occasionally happen that the sheaths bind the apex of the bud so tightly that the new growth of 
leaves is unable to force its way out. 
