1952] Linder,—Some Varieties of Panicum virgatum 11 
15? C., 20? C., and 25° C., or lines of the isotherms of these degrees of tempera- 
ture for the month of August. The broken lines running tn towards the shore 
are the isocrymes of 5? C., 10? C., 15° C., and 20° C., or lines of the isotherms 
for the month of February. "These two sets of isotherms show well the relation 
of Cape Cod to the average seasonal maxima and minima of the surface 
temperature of the waters of the coasts above and below it. I have pro- 
longed the isothere of 20? C. inward to the very coast itself and have attempted 
to sketch its deflection inward and northward as indicated by such data as to 
the temperature of the surface waters just offshore as are available. The 
deflection toward Long Island Sound and along the coast eastward to the 
shoals about Nantucket Island, thence northward to about Nauset on the 
eastern coast of the Cape Cod Peninsula, indicate the transition area lying 
between the North Temperate Zone above and the North Subtropical Zone 
below. In this transitional area, the outer coasts are of the North Temperate 
Zone while the inner are of the North Subtropical Zone. The dotted lines 
in Cape Cod Bay, in Vineyard Sound, and in Long Island Sound indicate 
that the isothere of 20° C. passes below the surface at these places. The 
deflection of the 20° C. isothere as sketched must be considered as only an 
approximate to accuracy in details. I have to thank my nephew, Charles 
= Davis, and Miss. Ruth Jeanette Powell for preparing the map for repro- 
uction. 
SOME VARIETIES OF PANICUM VIRGATUM. 
D. H. LINDER. 
THERE has been considerable difficulty in separating from Panicum 
virgatum L. its var. cubense Griseb. or var. obtusum Wood. Wood’s 
description! of the spikelet of the latter variety (from New Jersey) so 
closely matches the figure published by Hitchcock & Chase? of a 
spikelet from Grisebach’s type of var. cubense that there is no doubt 
that the two varieties are identical. By Hitchcock & Chase the species 
and variety are separated in a general way by the size of the spikelets, 
the stoutness of the culms, and the shape of the panicle. Very little 
Cuban material has been examined, but such as has been studied 
closely matches the North Carolina and New Jersey plant referred to 
var. cubense. The appearance of the panicle is quite marked, the 
rays being fewer and farther apart than in typical P. virgatum, but 
the best criterion for the separation of the two is the spikelet. In the 
variety the lower glume is less than half the length of the spikelet 
and is broad and blunt, the second glume and the palea are about 
equal in length and are slightly exceeded by the lemma. The floral 
parts usually are appressed, giving the spikelet a cylindrical outline. 
1 Wood, Botanist and Florist, 392. 1874. 
2 Hitchcock & Chase, North American Panicum. Contrib. U. S. Nat. Herb. 15: 
93. 1910. 
