1897 } NORTH AMERICAN SPECIES OF AMBLYSTEGIUM 237 
Such expressions as ‘‘ little longer than in the preceding,” 
‘shorter than in the former species,” etc., are of very little 
value in the description of any plant, since one man’s mole hill 
may be another’s mountain. For the gametophyte I have given 
the following measurements: length and diameter of stem; 
length of branches; diameter of central strand when present; 
diameter of cells in cortex; central strand and ground tissue; 
dimensions of leaf and leaf cells in three regions, alar, middle, 
and apical. Forthe sporophyte measurements were made of the 
length; the diameter of the seta, its central strand, and the 
cells of its three tissue regions; the length of the capsule; the 
dimensions of the exothecium cells and the stomata found on 
the collum. The averages for the leaf cells were obtained by 
measuring cells in each of the three regions from five leaves, 
taking five cells from each region of each leaf. To obtain the 
average dimensions of leaves a larger number were measured, and 
for averages of the exothecial cells and those of stomata from ten 
to twenty or more were measured, whenever there were so many. 
In general I have found that the characters of the sporo- 
phyte have comparatively little diagnostic value. There are 
Occasional exceptions. For instance, on the collum of the 
capsule of A, noterophilum there are from ninety to two hundred 
Stomata. These are commonly more or less clustered, as many 
aS seven contiguous ones in a single instance being found. In 
the nearly related species, A. fluviatile, the number reaches 
eighty, while in another related species, A. irriguwm, the number 
is usually below forty-five; and in both these species they are 
Scattered. 
We must still look to the leaves for the principal characters 
by which to separate the species. Their shape and size, the 
Size of the cells in the different regions, the presence or absence 
of a costa and its relative length and diameter are the most 
available marks. For this reason I have drawn illustrations only 
of leaves and parts of leaves, with a single exception of A. 
noterophilum. In this case there are no other figures of the 
capsule known to me. 
