186 BOTANICAL GAZETTE [SEPTEMBER 
Palisade tissue is developed to an unusual degree, there being 
at least two well defined rows of these cells, and often as many 
as four, in which case there is scarcely any spongiophyll and the 
air spaces are small and few. Where the spongy layer is prominent, 
there are groups of collecting cells at the lower ends of the deepest 
palisade cells. Protoplasm is abundant in all palisade and sponge 
cells, where chlorophyll is also present in large amounts. Crystal- 
logenous cells are found in small numbers. The upper epidermis is 
made up of medium-sized, lenticular, empty cells, which are regular 
and even in arrangement, and covered with a moderately. thick 
cuticle. The lower epidermis is composed of cells much less uni- 
form in size and shape, and in consequence the lower surface of the 
leaf is not so smooth as the upper. A few small hairs may be 
found widely scattered over the lower surface of the older leaves; 
hairs are quite numerous on younger leaves. The stomata are 
small, but very numerous, with the guard cells set flush with the 
lower surface. The midrib of the leaflet of box elder is very similar 
to that of A. rubrum (fig. 18), except that the crest of spongy 
tissue on top is even more prominent. 
In all of the maple leaves examined, there was found but one 
rank of palisade cells, and these formed not more, and usually much 
less, than half the thickness of the lamina (fig. 18). No well 
efined zone of collecting cells was observed. The spongiophyll 
contains many good sized air spaces, except in A. platanoides. 
The epidermal cells of both surfaces are quite varied in size, and 
the cuticle is thin. Stomata are comparatively few and some- 
what depressed from the surface. The midrib crest is present in 
all forms, but not so large as in box elder. Hairs are short and few 
on all types but A. platanoides, where they are quite numerous and 
long. A section of A. saccharinum leaf is shown in fig. 33. 
The petiole of box elder presents some interesting structural 
_ features (fig. 9). If we follow the three large leaf traces a little 
way up the petiole from their emergence from the stem, they are 
found to break up into a considerable number of fibrovascular 
strands arranged in an interrupted ring around a large medulla. A 
little farther out, not more than one-fourth of the way to the first 
pair of leaflets, there will usually appear from 1 to 4 or 5 medullary 
