PLANTS OF WESTERN LAKE ERIE. 
71 
Fig. F . — Bidens beckii, section through portion of 
aerial leaf. 
Steffi 
the tine-leafed Potamogetons the leaves are linear or narrowly lanceolate, sometimes 
almost capillary, while Vallisneria has long ribbon-like leaves that frequently attain 
a length of 6 feet. The broad submersed leaves 
of such species as Potamogeton perfoliatus , P. 
amplifolius , P. Zonchites , and P. lucens are very 
thin and consist in great part of but three cell 
layers, an upper and lower epidermis and a 
median layer, all of the cells having essentially 
the same character. 
This broad, thin leaf is an ideal one for 
making the most of the dim light, but it is not 
so well adapted to withstanding the motion of 
the water. To offset this, these broad leaves 
have ribs that are wanting in other submersed 
leaves, and in these ribs bast fibers are found 
which, according to Schenck, 1 are absent in all other submersed leaves. In Eaias 
flex'di* the leaf is but two cell layers in thickness except at the midrib, and in 
Elodea an upper and a lower epidermis alone make up the thickness of the leaf. 
A palisade parenchyma, so universal 
in the leaves of land plants, is entirely t ppor surface, 
wanting in submersed leaves, the chlo- 
rophyll being arranged in the epidermal 
cells and on the tangential as well as the 
radial walls (see figs. F and G, portions 
of aerial and submersed leaves of Bidens 
beckii). The absence of stomata from 
most submersed leaves and the thinness 
of the epidermal cell walls are to be 
expected in a medium where adaptations 
for controlling transpiration arc unnec- 
essary. 
The stem of the submersed aquatic 
with its central cylinder and reduced 
mechanical elements is well adapted to 
the movements of the medium, while 
the numerous and often large cavities 
supply an abundance of air to all parts 
of the plant. The reduction of the vas- 
cular bundles is one of the most striking 
features. In some aquatics, as Edicts, 
no vascular bundles remain, their place 
being taken by a central canal (fig. Iv). 
This canal is also present in the Potamo- 
getons and in some dicotyledons, as Ceratopliyllum (pi. 13, fig. 5), and represents the 
wood portion of the bundle, an occasional ring or spiral indicating the former 
1 Schenck, H. Vergleichende Anatomic cler Submersen Gewachse. Bibliotheca botanica, Heft 1, 67 pp.,1886. 
Lower surface. 
Fig. G . — Bidens beckii, submersed leaf. 
