1897 ] NORTH AMERICAN SPECIES OF AMBLYSTEGIUM 273 
four layers of incrassate cells 4 to 17m in diameter; central 
strand well differentiated, 30 to 45 w in diameter, of twelve to 
twenty-one rows of cells; branches in terrestrial form ascending, 
I to 2.5™ long, those of the submerged form 3 to 8™ long 
bearing few short(1™) branchlets, in all cases densely leafy, the 
stems and branches of the submerged plants at length naked or 
covered with the very stout excurrent coste of the leaves, due to the 
maceration of the leaf blade: leaves varying from broadly trian- 
Sular cordate ovate in land form to long lanceolate in submerged 
form, generally abruptly short acuminate by the excurrent costa, 
straight or with the point moderately curved, in water form 5504 
wide by 13504 long, varying from 450 to 700m wide and from 
1050 to 1500 long, extremely large plants having leaves goo 
wide by 22004 long, in terrestrial plants the leaves proportion- 
ately wider, 500m wide by 850 long, rarely reaching 450 wide 
by 600 long, strongly costate, costa commonly excurrent forming @ 
very thick rather blunt cusp, sometimes in the land form vanishing in 
the tip, one-sixth to one-third the leaf base at line of insertion, 
65 to 225 wide, plano-convex at base, double convex above to 
the tip, there sometimes becoming again flattened on the upper 
Side; leaf blade very commonly of two layers of cells in the basal 
sixth and along the costa well toward the tip, margin usually entire, 
rarely slightly serrulate in the upper part of blade, greatest 
Width of leaf attained in the basal sixth, open or erect spreading 
in land plants, slightly spreading or strict in submerged form ; 
leaf cells parenchymatous at base, prosenchymatous in the middle 
and upper lamina, or mixed at the upper end of the blade, in the 
alar and apical regions in the extreme long and short forms 
Cells essentially the same in size, those in the middle portion of 
the long leaves proportionately longer, in average leaves alar 
cells 14m wide by 22m long, varying from 12 to 17m wide and 
from 19 to 23m long, the middle leaf cells varying from 9 to 
Tl“ wide and from 38 to 45m long, averaging 10.5 wide by 
43-4 long, these cells varying in the extreme forms from 9 
by 18m to 12 by 90p, cells in upper lamina beside the costa 
TOM wide by 28 u long, ranging from g to 11m in width and from 
