70 THE RELATION OF PLANTS TO TIDE-LEVELS 
0.5 meter long were found just at the edge of the main stream at 10 north by 
600 east where it is washed by fresh water for 2 or 3 hours each tide. 
From these examples it is evident that while Ascophyllum will. not withstand 
constant submergence in fresh water, it can endure submergence in it for 2 or 
3 hours each tide, provided that it is immersed in salt water for the rest of the 
tide. On parts of the wharf walls not exposed to fresh water the denser growth 
of Ascophyllum usually ceases at about 5.5 feet, but odspores germinate at con- 
siderably higher levels in protected places between rocks or under the shade of 
the more hardy Fucus. As these young Ascophyllum plants grow, however, 
and push out from under these protecting objects, they are subjected to severe 
desiccation during the long exposure to the air. Most of the plants that have 
started at these higher levels are killed off, and those that persist are few and 
dwarfed. The highest plants of Ascophyllum found were always in places 
protected from extreme desiccation by northern exposure or overshadowing 
rocks, piles, or Fucus. It seems clear that the upper limit of distribution of 
Ascophyllum is determined by the time of exposure to the air, though it is true 
that Ascophyllum, like Fucus, may endure drying out until it becomes brittle 
enough to crush, almost to powder, in the hand. 
The lower limit of Ascophyllum on the wharves, which is usually at 1 foot 
or more above the bottom of the wall, is probably determined by the injury, by 
burial in the soft mud, to those plants that grow nearer the bottom than this. 
Where the bottom is stony, as at 2,000 to 2,600 north, on the east side, we have 
already noted that the Ascophyllum goes to the extreme lower edge of the wall 
and then continues on over the stones of the bottom down to mean low water, 
below which it is very rarely found. (See plate xviir A.) 
Fucus: The two species found are not distinguishable by grosser characters 
that can be seen as the plants hang on the wharves. We shall therefore simply 
record the distribution of the genus without attempting to distinguish the 
-species. The plants of Fucus found vary in size from young sporelings to 
plants 2 or 3 dm. long, and with main branches 15 or 20 mm. broad (plate 
xvi11B). They occur on all the substrata bearing Ascophyllum, that is, on 
stones, shells, and piles. Fucus, like Ascophyllum, is distributed almost con- 
tinuously along nearly all the wharves about the harbor, but is not present on 
the yellow granite blocks of the wall of the east side to which we have already 
referred (plates xvir and 1118). The vertical distribution of Fucus is also 
similar to that of Ascophyllum, except that the latter is dwarfed, much less 
abundant, and often quite wanting in the upper foot of the rockweed belt, which 
belt, above 5.5 feet, is dominated by Fucus. In the more exposed places the 
Fucus is usually quite unmixed with Ascophyllum. 
Below the 5.5-foot line of the rockweed belt fucus occurs commonly as 
isolated plants or in groups of 3 or 4 plants scattered among the Ascophyllum. 
Only occasionally is a patch of Fucus as much as 0.5 meter square found at 
these lower levels. Within the zone where both Fucus and Ascophyllum grow, 
the latter apparently conquers wherever its odspores can find space to settle, 
and light enough to allow the young plants to start. The upper limit of Fucus 
moves upward on wooden wharves and on shaded stones or piles. On the wooden 
wharf at 2,230 north, on the west shore, the planks are not quite horizontal and 
the highest Fucus found is attached to the edges of these planks, at 6.75 feet, 
where water oozes out slowly after the fall of the tide below this level. It does 
