516 Wisconsin Academy of Sciences , Arts, and Letters. 
a very dense and luxuriant growth on tumbled masses of sub¬ 
merged or half-submerged rocks; a similar dense growth on rocks 
scattered much more thinly over the bottom; and a scanty growth 
on small pebbles or single, isolated large rocks. 
In the case of the first of these types (Type A, table 11), 
samples were collected from individual rocks, a whole rock being 
cleaned off and the area covered by Cladophora measured. From 
a number of such samples, the weight per square meter of rock 
surface was arrived at. By cleaning off all the rocks from a small 
section of ground, and by then measuring the area of this ground 
and the total surface of all the rocks that came from it, the rela¬ 
tion of ground surface to rock surface was determined. And by 
comparing the average ratio of the surface of a rock to the amount 
covered by Cladophora, it was possible to estimate roughly the 
relation between a measured area of ground and the Cladophora- 
bearing surface upon it. On one square meter of ground, there 
were nearly 2 square meters of Cladophora-bearing surface. Twice 
the weight per square meter of rock surface is therefore an ap¬ 
proximation of the weight per square meter of ground surface. 
The second type (Type B, table 12) includes samples of very 
different values per square meter. Since these differences would 
not be readily distinguished in the field, so that the relative area 
occupied by each sort could be determined, it was necessary to 
group them all together, and to obtain an average. Samples were in 
five cases collected from measured areas of ground. Three came from 
rocks of average character, representing a large area in which the 
total number of rocks was counted. The first two samples shown 
in the table came from station 1, which, however, is usually char¬ 
acterized by growth of Type A. The explanation is that, as the 
main crop of Cladophora dies down toward the end of June, the 
water of the lake rises slightly, and on the rocks now for the first 
time submerged a new crop arises. This, however, is not of so 
luxuriant a character as the earlier growth, and for this reason 
is not classed under Type A. 
Type C, representing those places where there is a very scanty 
and scattered growth of Cladophora, is illustrated by one sample, 
shown in table 13. From this sample the total weight of Clado¬ 
phora of this type—relatively a very small quantity—was cal¬ 
culated. 
Cladophora is a significant element in the lake vegetation dur¬ 
ing only a small part of the growing period. Early in June, the 
