CHIMNEY-SHAPED STOMATA OF BURRO THORN. 
I27 
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 (h), 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- 
Fig. 30. A, part of a transverse section of a twig; B, section parallel 
to surface and near to the palisade tissue (seen at the right) ; C, a simi¬ 
lar section near to the cuticle. 
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. 
Cross-sections of the stomal cavity (chimney) show that at its 
outer end it is round or elliptical, and of somewhat smaller diam¬ 
eter than elsewhere. Below the top of the chimney the cavity 
is round, elliptical, ovate, or irregularly oblong in section (cy). 
The walls of the chimney are composed of about eight vertical 
