72 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
presence of vessels. In Bidens beckii the dicotyledonous type of bundle is found, but 
the bundles are small and far apart. Between the bundles and the epidermis there 
are numerous cavities separated from each other by layers of tissue one cell in thick- 
ness (pi. 13, tig. 1). In Oeratophyllum the dicotyledonous type of stem lias been 
obliterated and the entire stem is composed of parenchymatous cells, those of the 
central cylinder being only slightly differentiated. The stem is strengthened by 
collenchymatous thickening of the angles of the cell walls (pi. 13, fig’. 6). 
The Potamogetom having the monocotyledonous type of bundle differ from 
Bidems beckii in the central cylinder, but outside of this central cylinder the two 
forms have a similar structure. A sheath of thick- walled cells surrounds the central 
cylinder, and such cells are also frequently clustered about the inner side of each 
bundle. In some flat-stemmed Potavioqe- 
tons , as P. zoster ecefolius, groups of thick- 
walled cells are placed at irregular inter 
vals just beneath the epidermis (pi. 13, 
tig. -1). 
The structure of the floating leaf shows 
many striking differences from the sub- 
mersed leaf, due to the difference in envi- 
ronment. The cells of the upper epidermis 
of the floating leaf are smaller than in the 
submersed leaf, with thicker outer walls, 
and frequently of more irregular outline. 
Stomata are confined to the upper epider 
mis. Below the epidermis are one or two 
layers of palisade tissue with the chlorophyll 
arranged on the radial walls. Between the 
palisade tissue and the lower epidermis there 
are large cavities separated by partitions 
one cell-layer thick (figs. C, E). 
The lower epidermis is composed of 
larger, thinner-walled cells than the upper epidermis, and is devoid of stomata. 
Floating leaves are of firmer texture than submersed ones and have some protection 
against injury by water. This protection in most aquatics is a waxy covering 
(Schenck), but in Nelumbium luteum it consists of countless papillae, each arising from 
an epidermal cell. A layer of air is always held by these projections, so that water 
falling’ on the leaf stands in great drops, as if on an oiled surface, until it can run off. 
Nelumbiuvn luteum has both floating and emersed leaves. There is no essential 
L/rTW 
Fig. K . — Naias flexilis, cross section of central cylinder. 
