48 
Journal of Agricultural Research 
Vo!. XXVI, No. * 
the excrescence, although differing from it histologically. It is in this 
region that the greatest hypertrophy occurs. 
. 0n the exterior of these galls may be found pieces of the original smooth 
rind of the tree which had been tom loose and pushed outward by the 
pressure of the proliferating tissue within. The external portion of the 
tumor tissue soon dies and dries up, becoming hard and brittle. Nor¬ 
mally but little gum exudes from the galls, but when they are removed 
from the tree and soaked in water or glycerin great quantities of gum 
readily diffuse out. 
NORMAL HISTOLOGY OF THE MOORPARK APRICOT 
If a cross section is made through a healthy old Moorpark branch and 
the tissues are followed from the exterior inward, it is found, to begin with, 
that the epidermis is ruptured and tom owing to the stress of internal 
growth, and that beneath it there is a relatively wide area of seemingly 
stratified tissue (PI. 5, A, B, C,) consisting of layers of flat, tabular or 
brick-shaped cells. These cells are filled with air, and therefore have a 
refractive index differing from that of any other tissue. By means of 
these characters and also by the close arrangement of the cells, allowing 
no intercellular spaces, this tissue is readily recognizable from any other 
in the plant system. This is the cork, which, being impervious to water, 
serves the plant as a protection against excessive evaporation and ex¬ 
treme changes of temperature. Alternating with these cork strata may 
be found strata of thin-walled cells wider in radial diameter than the 
cork cells, and angular rather than brick-shaped. This is the cork 
parenchyma or phelloderm (PI. 5, D, E, F). Scattered through these 
tissues, and more abundantly through the cork parenchyma, may be 
seen clusters of cells whose walls are greatly and uniformly thickened; 
these are sclerenchyma—or stone cells (PI. 5, G, H), which serve as a 
buttressing element to the tissues in which they are found. 
These tissues arise from the cork cambium or phellogen, which in the 
genus Prunus f to which the apricot belongs, originates in the subepi- 
dermal layer of cells (<£, p. 307) . 3 The primary cork cambium does not 
remain active indefinitely, and a new one is formed from the innermost 
layer of cells recently laid down by it. This new cork cambium form9 
cork tissue toward the exterior, and, less frequently, thin-walled paren¬ 
chyma, or phelloderm tissue, toward the interior. This explains the 
occasional occurrence of layers of phelloderm between cork strata (PI. 5). 
All of these tissues—cork cambium, cork, and cork parenchyma—located 
between the epidermis and the phloem or bast region underlying them 
(PI. 5, I), are collectively spoken of as periderm. 
It will be noted that the line of demarcation between the periderm and 
the phloem is sharp. Rarely, if ever, do we find in the normal apricot 
plant, cork tissue extending any appreciable distance into the phloem. 
While small portions of the outermost zone of the primary phloem may 
occasionally be intercepted by cork layers, the successive strata of cork 
tissue tend, as a general rule, to preserve their parallel course; the 
divergent cork strand soon rejoins the tissue from which it arose. 
In this feature—namely, the sharp demarcation between phloem and 
cork tissues—lies the essential difference between the histology of the 
normal apricot tree and that of the galls. 
• Reference is made by number (italic) to “Literature cited,” p. 59. 
