176 BOTANICAL GAZETTE [SEPTEMBER 
walled cells for the greater part of its thickness,.with a few layers » 
of collenchyma toward the outside, covered by a small-celled 
epidermis over which is spread a smooth, uniformly thick cuticle. 
Sclerenchyma tissue occurs as a narrow and frequently inter- 
rupted band of bast fibers, with numerous small and irregularly 
scattered masses of stone cells. The bark contains some starch, 
and usually a few large empty cells, corresponding to the sap- 
storage cells of the root. No true phellem is formed in the first 
season. 
Comparing figs. 19 and 17, it will be seen that the leaf traces 
of the maple are inconspicuous, the medullary starch zone is not 
prominent, medullary rays are less numerous, and the bark is 
thicker and more dense. In twigs of Acer rubrum the starch zone 
is thinner; there is less sclerenchyma; no phellem, but many thick- 
walled tanniniferous cells; cuticle thick and lenticular over a 
larger-celled epidermis. In A. saccharum and A. platanoides, the 
starch zone is thin; few medullary rays; but little sclerenchyma; 
phellogen and phellem very prominent, with numerous large 
lenticels; epidermis early tanniniferous; cuticle thin and smooth. 
The medullary rays of box elder are continued far out into the 
cortex, both in young and in older stems. A study of figs. 13-16, 
which show sections of older stems, will make clear the relative 
development of box elder in this respect. Further comparison with 
figs. 11 and 12 indicates that this prominent development of the 
medullary rays in the cortex of box elder is a primitive ancestral 
characteristic, as well as another important factor making for rapid 
growth. 
An additional point of interest in this connection is found in the 
form and number of the pits in the walls of both the ray cells and the 
cells of the amyliferous zone. While these pits are comparatively 
small in the box elder, they are far more numerous here than in any 
of the true maples. Moreover, they are ideally arranged to facili- 
tate the movement of sap to the regions of most active growth. 
For comparison of sizes of the various cells, and the thickness of the 
walls, the reader is referred to table I. In general, it may be 
said that the tissues of the young stem of box elder are much less 
compact than in the case of the true maples. 
