58 
THE SEAWEEDS 
transpiration or for preventing evaporation. Their outer surface is not 
protected by any special cuticle or corky layer; there are no stomata for 
the control of air supply. The buoyancy of the sea water, heavier than 
fresh water, reduces the necessity for undue expenditure in strengthening 
the tissues of the supporting steins. 
The structure is then parenchymatous. The cells in general form a 
cortical coloured assimilative stratum and a medullary or central colourless 
supporting and conducting stratum, the former, often included in mucus, 
causing the slimy feel of many seaweeds. The cells of this layer are usually 
small, often minute, arranged in a single layer or in simple or branching 
strings. The mucilaginous character of the cortical tissues of many algae 
protects the internal cells from drying, while temporarily exposed between 
tides. Growth in length and width is either apical, effected by divisions of 
a single apical cell, or of a marginal series of cells, or from a more extensive 
meristematic group ; or is intercalary, as in Macrocystis where from a 
meristem at the junction of stalk and blade additions are made to both, in 
opposite directions, much as bark and wood on the two sides of the cambium 
layer in vascular stems. 
Vesicles or Floats. — Several of the Brown Seaweeds, of the Families 
Sargassaceae, Fucaceae, and Laminariaceae , form bladder-like organs which 
contain gas. These, especially in the genera Sargassum and Cystophora, 
are berry-like and are consistently regarded as fruits by the casual observer, 
but their purpose is quite other than reproduction. They serve as floats to 
assist the support of the frond in a vertical position. The walls of the 
floats are solid and continuous, impervious to air or water, and there are 
no canals connecting the gaseous interior with the outside medium. The 
floats vary in size in the various species, and also grow from small 
beginnings in the same individual. In Sargassum and Cystophora they are 
rarely larger than a pea ( Pisum ) and terminate stalklets. In Phyllospora 
and Macrocystis they are oval and are formed in the lower parts of the 
fronds; in Phyllospora they are about one inch and in Macrocyst es four 
to five inches long. In Nereocystis, a North American Pacific plant, with 
heavy fronds 40 feet or more in length, the vesicles are gigantic, each having 
a final gaseous capacity of a litre. 
The gas of the floats is derived from the gases “dissolved” in the sea 
water. Usually it is found on analysis to consist of nitrogen and oxygen 
only, but not in the proportions in which they occur in atmospheric air, 
still less in the proportions in which they occur in the gases obtained by 
boiling sea water. In common air we have 79 per cent, of nitrogen to 21 
per cent, of oxygen with traces of carbon dioxide. In the gases obtained 
from sea water there is more variation especially in the amount of carbon 
dioxide present. A typical analysis of the gases of ocean water from off 
Coogee on the coast of New South Wales gave in 100 vols. of sea water gas, 
nitrogen 58.33, oxygen 30.55, carbon dioxide 11.11. 
