22 Raymond John Pool 
different species, and yet there is not a species of Juncus described 
from the region for the Artemisia formation. Surely they are 
two different species viewed from the histological structure, but 
from the taxonomic standpoint they are apparently the same, and 
we have here the extremes of histological differentiation within 
the species. 
The wet-meadow form (fig. 41) shows a thick epidermis with 
a very thin cuticle and few stomata. The chlorenchyma consists 
mainly of two rows of palisade cells closely aggregated, besides 
a single row of globose cells just beneath the epidermis. The 
palisade cells have a tendency to radiate from the vascular bun- 
dles. There is one large central vascular bundle with a peripheral 
chain of about 15 to 16 smaller bundles immediately beneath the 
palisade. On the interior and connecting all the bundles is an 
open reticulum of cells which forms the boundaries of the inter- 
nal aeration chambers. The individual vascular bundles show a 
fair development of transportive elements or xylem with a slight 
development of supportive elements or fibers. The bundles lack 
a well-defined sheath of wood fibers which is so characteristic for 
the monocotyledonous vascular bundle. 
In the dry form we find that the cuticle is three times as 
thick as in the above stem, it having filled about one-half of the 
cell cavity. The number of stomata is greatly increased, there 
being twenty on the section here to six on the other form. From 
actual counts for equal areas the ratio of the number of stomata 
in the two forms is 10 moist form to 30 dry form. In this stem 
the chlorenchyma is about doubled over its moister neighbor, and 
it is so compact that it is almost impossible to see the intercellular 
air spaces. The center of the stem is at first made up of a uni- 
form tissue of globose cells which breaks down as the stem gets 
older and finally leaves a central longitudinal cavity as shown in 
the figure. 
The most noticeable difference, however, is in the number and 
the structure of the vascular bundles. These are disposed 
in the form of three more or less regular peripheral rings. The 
outermost ring is composed of small, inconspicuous bundles much 
like those in the meadow form. There are about twelve of these 
432 
