66 THE RELATION OF PLANTS TO TIDE-LEVELS 
It is evident that the rockweed covering of the wall may be pretty completely 
renewed by this sort of regeneration each year, if we assume that the rate of 
growth found by Miss Streeter for July is continued for most of the year. In 
careful measurements of the elongations of plants of Fucus varying from 1 to 
20 cm. in length, made daily for a month or more, Miss Streeter found an 
average daily growth varying from 0.5 to 1 mm. in different plants. Observa- 
tions on the rate of growth of sporelings, however, as determined by the 
measurement of the same young individuals at periods 3 months apart, and by 
the time taken to cover a piece of new wall, indicates that the covering of the 
wharf near the Laboratory could not be renewed in a year’s time. 
With the above-noted general characters of the rockweed association in mind, 
we will now discuss the individual species of which it is composed, in the 
systematic order indicated on page 64. 
SCHIZOPHY CE. 
Most of the members of this group found in the rockweed association occur 
in small blackish felts or gelatinous patches, occasionally on stones, but chiefly 
on piles or dock logs. The peculiarities of each species of this class are indicated 
below. 
Calothrix fusco violacea occurs in blackish felts 1 or 2 cm. wide, several 
centimeters long and 1 to 2 mm. thick. This has been found rather generally 
about the harbor between the 6 and 8 foot levels, on stone (as at 1,050 north by 
450 west at 6.5-foot) or on wood (as at 2,200 north by 1,230 east near the 7.5- 
foot level). At these levels it is evident that the plants must withstand long 
exposure to desiccation and to rain, the last habitat mentioned above being the 
high-water line of neap tides. It is noteworthy also that this species seems to 
grow higher on wood, which has the capacity for absorbing and conducting 
water more readily, and therefore probably keeps the algee more moist when 
above the water-level than they would be on stone. Another Calothriz, which 
~ resembles C. scopulorum in the size of its cells, was found under the mill at 
500 north by 1,000 east at the 6 to 7 foot levels, on a wooden post, wet by a spray 
of fresh water at low tide and by salt water at high tide. Calothrix crustacea 
prolifera occurs between 7 and 8 feet on logs of the Research Laboratory Wharf. 
Lyngbya sp. was found on the post just referred to above. 
Lyngbya (sp. 2 to 4.54 in diameter, with tortuous tips) formed felts 1 mm. 
or more thick on the same post and on piles and pieces of wreck between 1,100 
and 1,230 north on the west shore at the 6.5 to the 7 foot level. 
Oscillatoria (at least two species) was found at the 7-foot level on the wharf 
of the Research Laboratory, on the wreck just south of it, and also on the bell 
of an hydraulic ram operated by fresh water near 2,830 north on the west shore. 
Rwularia (sp.?) formed dozens of blackish gelatinous disks 0.5 cm. in 
diameter on logs of a wreck at 1,240 north by 575 west, between the 6 and 7% foot 
levels. 
These few blue-green algee were not especially sought for on all the wharves 
about the harbor, and hence might possibly have been found elsewhere in some ~ 
seasons. Perhaps each of them may be found to be generally, though sparsely, 
distributed on similar substrata at similar levels. Several of them, in fact, 
were found on the beaches between the same levels. We are clearly safe in 
concluding that the species of these simple alg are confined, like most other 
