178 BOTANICAL GAZETTE [SEPTEMBER 
Considering now more specifically. the minute anatomy of the 
older trunk of box elder, we find that the rough ‘“‘expanded metal” 
appearance of the trunk surface shown in fig. 2 is almost exactly re- 
produced in the photomicrograph of a tangential section of the 
bark (fig. 24). The numerous concentric layers of bast fibers 
are imbedded in a spongy mass of soft, thin-walled paren- 
chymatous phloem, which is split up into thin radial plates by the 
medullary rays. Near the cambial zone these layers of hard bast 
are almost continuous, except where perforated by the thin medul- 
lary rays. As the sheet of hard bast is forced outward by growth, 
the fibers cling tenaciously to each other, and the whole sheet is 
expanded into an oblique diamond-mesh pattern, as a result of the 
great tangential tension. At the same time, centripetal pressure 
is brought to bear upon all of the tissues inside the network of 
bast fibers. As a result of these conditions, there is more or less 
radial compression and tangential expansion of all the softer ele- 
ments of the bark. For the same reasons, there is likely to be a 
considerable tangential shifting of each layer of hard bast with 
reference to that next inside. When this shifting is all in the same 
direction in all the layers, the medullary rays are often bent aside 
30° or 40° from the radial line. This is of course a common feature 
in those plants in which the medullary rays extend far out through 
a thick bark. The hard bast fibers are quite slender, except occa- 
sional isolated specimens, but they are usually several millimeters 
long, and very firmly united with each other. The walls are very 
thick, often almost eliminating the cell cavity. Angular, irregu- 
larly shaped sclerotic cells are found in fairly large numbers, most 
frequently in the angles of the bast fiber network. In the young 
bark the medullary ray cells are oval in form, and occur in loose 
moniliform rows. The nuclei are here of large size and the proto- 
plasm is especially abundant. The medullary ray cells in the outer 
part of the live bark are shorter and thicker. As seen in tangential 
section, these cells are commonly drawn out transversely into an 
oval shape. In the still older bark the ray cells are empty, crushed 
and distorted by the pressure of growth. The bast parenchyma 
begins its development in the form of thin-walled, radially com- 
pressed cells about the diameter of autumn wood cells. From 
these are derived two types of parenchymatous cells: the larger 
