15 
tissue. The latter were hexagonal in cross, and rectangular in longi¬ 
tudinal section, Figs. 1 and 2. Their walls are rather thick and con¬ 
tain numerous thin places which are elliptical in form and lie with their 
long axes perpendicular to the axis of the stem. Many of these seem 
to have broken through and formed openings from one cell to the next. 
The intercellular spaces are small. The size of these cells varies with 
their position, those midway between two bundles being larger than 
those immediately adjacent to them. 
The bundles are scattered through the ground tissues at somewhat 
regular intervals, always with the phloem turned toward the epidermis. 
Near the periphery of the stem they are packed more closely together, 
with very little of the ground tissue between, Fig. 3. They are of the 
form kuown as the closed collateral bundle. Within the xylern and 
toward the inner side of the bundle is a space, Fig. 4, a. Partially sur¬ 
rounding it is the xylem parenchyma, which is composed of small ele¬ 
ments with thinner walls than are found in the adjacent tissue. Back of 
this canal and partially projecting into it is an annular vessel, b. At 
each side of the bundle is a large duct, c, with thick reticulated walls. 
Between the ducts and back of the annular vessel are thick-walled 
elements, the tracheides. Between the tracheides and ducts and the 
external part of the bundle is the phloem d, consisting chiefly of sieve 
tubes andcambiform cells. The latter, small, thin-walled, square,or rect¬ 
angular, sometimes narrow or obliquely four sided elements, are dis¬ 
tributed among the sieve tubes, which are rather large, thin-walled, 
polygonal elements. Both sieve tubes and cambiform cells are filled 
with a granular protoplasm. The elements of the bundle sheath e 
have thick pitted walls. It is developed best at the ends, particularly 
at the outer end, where it may consist of several layers. At the sides 
it often borders directly on the ducts. In longitudinal sections, Fig. 5, 
all of the elements are found to have transverse septa, except those of 
the bundle sheath, in which they are oblique. The sieve tubes are longer 
than the cambiform cells, and have the characteristic callous plates. 
The annular vessels are seen to consist of a very thin-walled portion, 
which is held open by thick rings placed at nearly regular intervals, g. 
The bundles just under the epidermis are usually somewhat different 
from those just described in that the larger intercellular space is par¬ 
tially or wholly absent. 
The epidermis is made up of cells, which are represented in cross sec¬ 
tion in Fig. 3, u, in longitudinal section in Fig. G, a, and in surface view 
in Fig. 7, a. As seen in the two latter they are elongated cells with 
slightly sinuous outline and thick pitted walls, the outer of which is 
somewhat the thicker and covered with a thin cuticle. In longitudinal 
sections and in those which show surface views there are seen to be two 
kinds of cells, those already described and short cubical cells. The 
latter are found usually one at each end of a long cell, but sometimes 
there are several of them together. The sub-epidermal tissue consists of 
