1904] BILLINGS—TILLANDSIA USN EOIDES 119 
the pressure of the water-storage tissue exert any influence on the 
opening and closing of the stomata it is very probable that the injury 
done to the tissue in sectioning would greatly interfere with the action 
of the mechanism. 
HABERLANDT (8) figures the stoma of Ti/landsia zonata, which in 
respect to guard cells, and their supporting cells, resembles that of 
T. usneoides. The guard cells have greatly thickened walls, and a 
thickened hinge. From Haberlandt’s account it is evident that he 
does not fully comprehend the mechanism. In 7. zonata no subja- 
cent parenchyma is mentioned as taking part in the opening or closing 
of the stoma. 
THE STEM. 
Aside from the vascular region, the stem differs in no essential 
particulars from the leaves as to structure. The stem, of course, has 
the added function of support, so that there is developed between and 
around the bundles a thick tissue of sclerenchyma fibers (fig. 74). 
The fibers measure about 750 in length. They do not impart 
rigidity, but flexibility and power to resist longitudinal strain. If a 
fragment of moss is blown from one limb of a tree to another, and 
succeeds in getting a hold, the cortex of that portion of the stem that 
passes over the limb dies, and then disintegrates, leaving the scleren- 
chymatous axis, which holds the plant in place for several and perhaps 
many years. It is upon the durability and elasticity of this tissue 
that the economic value of the moss in upholstery depends. 
What has already been said in regard to reduction in.the function 
of the xylem and phloem of the leaves could with equal truth be said 
about the stems. With a superficial absorptive system and no root, 
the xylem as a conductive system is useless. The pendent habit and 
method of dissemination are both closely associated with reduction in 
mechanical tissue, though they are more likely to be the result than 
the cause of the reduction. The parenchymatous cortex, as in 
leaves, is supplied with chlorophyll-bearing cells, all of which are 
exposed to light, so that a tissue like the phloem, to carry elaborated 
materials to cells distant from the center of photosynthesis, would be 
unnecessary. 
LovIstIana STATE UNIVERSITY, 
Baton Rouge, La. 
