90 
FLORIDA STATE HORTICULTURAL SOCIETY. 
the leaf consists of one layer of cells, 
known as epidermal cells. Just beneath 
the epidermis lie two layers of cells that 
are much longer than broad, and are ar¬ 
ranged as so many columns standing side 
by side, giving the appearance of a pal¬ 
isade. These cells are known as the 
palisade cells. Beneath these there are 
several layers of loosely arranged cells. 
These form arches and columns, leaving 
considerable air-spaces in between them. 
These cells constitute the spongy tissue of 
EXPLANATION OF FIG. i. 
Cross Section of Healthy Leaf. (A) Upper Epi¬ 
dermis, (B) Palisade Cells, (C) Spongy Tissue, 
(D) Lower Epidermis, (E) Air Spaces, (N) Nuclei. 
the leaf. Beneath the spongy tissue is 
the lower surface of the leaf, which con¬ 
sists of one layer of epidermal cells, with 
openings for air, etc. 
Cross-sections of the yellow spots, 
when studied under the microscope, show 
the cells of the spongy tissue to be so 
swollen that the air-spaces between the 
cells are completely filled, Fig. 2. This 
swelling also causes a pushing out of the 
lower epidermis making the spot project. 
A brownish gum-like substance occurs 
at intervals between the cells. This is a 
secretion from the cells. In Fig. 2 this 
substance is shown in the shaded portion. 
A brownish deposit also fills the upper 
epidermal cells. It is this that gives the 
dark color to the center of the spot on 
the upper surface of the leaf. 
Corky tissue also develops, particularly 
in late stages of the spotting. It is the 
formation of this cork that gives the 
areas their melanose-like appearance. The 
corky tissue usually develops only on the 
lower surface of the leaf. It arises by 
division of the epidermal cells, Fig. 1. 
The corky tissue may also develop on the 
upper epidermis, taking its origin by di¬ 
vision of the upper epidermal cells. 
OTHER CASES OF YELLOWING OF CITRUS 
LEAVES. 
There are several different yellow ap¬ 
pearances on citrus leaves that may be 
confused with the yellow spotting. The 
yellow spotting differs in gross appear¬ 
ance from these by its dull greasy look 
and slight projection on the under sur¬ 
face, Fig. 2. 
The most common yellowing with 
which the yellow spotting may be con- 
