IOWA ACADEMY OF SCIENCES. 
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Mesophyll — This surrounds the bundles of the secondary type 
and occurs between all bundles. It is made up of irregular 
cells, but quite uniform in size. The chlorophyll granules 
are quite large and numerous. 
BROMUS PATULUS, M. & K. 
(PI. V., Fig. 3; PI. vi. Fig. 6.) 
This is a small early form determined as B. nivilis. 
Epidermis . — The epidermal cells of this species are large, 
regular, thick- walled with a strong, well developed cuticle; the 
cells above and beneath the carene are smaller and thicker than 
elsewhere; the leaf is more involute than that of any other 
species studied, unless possibly Bromus racemosus. Trichomes 
are numerous, — some very long and slender, others are short 
and thick. Stomata occur on both surfaces of the leaf. 
Bulliform cells . — The bulliform cells vary in number from 
three to five, and are not as apparent as in some of the other 
species studied. These cells occur on superior surface of leaf 
between the mestome bundles. 
Mestome bundles . — These number from twenty- five to thirty 
and are of two kinds. The bundles of the primary type num- 
ber from nine to eleven, represented by the principal bundle of 
the carene. This is open both to the upper and lower surfaces 
of the leaf, i. e. , the leptome is in direct contact with the stere- 
ome, while the hadrome is separated from it only by colorless 
parenchyma cells. The secondary bundles number from six- 
teen to eighteen. The leptome and hadrome are entirely sur- 
rounded by chlorophyll- bearing parenchyma. In the largest 
bundle of the primary type the stereome is very abundant, while 
in the smaller ones, it is reduced in some instances to a single 
row of cells; the leptome and hadrome are well developed in 
these bundles and they are separated- from each other by-thick- 
walled cells resembling stereome. Both spiral and pitted ducts, 
as well as the intercellular space, are well defined. The secon- 
dary bundles are surrounded by colorless parenchyma without 
stereome. The leptome and hadrome are differentiated. Two 
secondary bundles occur on margins of leaf. 
Carene . — The carene consists of one typical primary bundle 
with leptome and hadrome well developed and separated from 
each other by thick- walled parenchyma cells; the pitted and 
spiral ducts are well developed and also the intercellular space 
is conspicuous. On one side of this bundle is another primary 
