Bessey: STOMATA OF HOLACANTHA 525 
All parts of the surfaces of the branches and thorns are of a 
pea-green color, and minutely roughish to the touch. Upon 
making a transverse section the epidermis is found to be of un- 
usual thickness, being no less than three, four, or five layers deep. 
The outer wall is of excessive thickness, and those below, while 
much thinner, are. still very thick. The lateral walls, also, are 
greatly thickened, so that in a superficial view the cell-cavities are 
widely separated. Microchemical tests show that the outer epi- 
dermal cell-walls are strongly cuticularized, while those below are 
less so, or not all. 
Below the epidermis lies a thick mass of palisade cells (f), 
averaging 110 in thickness, abundantly supplied with chlorophyl. 
These cells are closely packed in about three layers, the cells 
averaging from three to four times as long as their diameter. In 
this palisade mass there occur at frequent intervals large crystal- 
cells, each containing one large clustered crystal (c). The tissue 
beneath the palisade cells is thick walled (%), but the walls are 
not cuticularized, the stain reaction being that characteristic of 
cellulose. . 
The stomata are thickly scattered over the surface of the stems 
and thorns, a careful count showing that they number about 
Seventy-three per square millimeter. Each stoma lies at the bot- 
tom of a narrow chimney-shaped cavity (cy) which passes entirely 
through the thickness of the epidermis and is prolonged both 
above and below it. This stomal channel is thus of greater 
length than the thickness of the epidermis. The measurements 
are as follows: 
Average thickness of the epidermis. hes 
Average depth of chimney-shaped channel. ses bil 
Average height of chimney above the ites? of the epidermis. 43 
Average a of the stoma proper below the inner boundary of 
the epidermis iia 
Cross-sections of the stomal cavity (chimney) show that at its 
Outer end it is round or elliptical, and of somewhat smaller di- 
ameter than elsewhere, Below the top of the chimney the cavity 
is round, elliptical, ovate, or irregularly oblong in section, and has 
an average diameter of about 25 » (cy). The walls of the chimney 
are composed of about eight vertical rows of cells (4c), whose 
