370 DR. SARAH M. BAKER AND MISS M. H. BOHLING ON 
based upon personal observations in the field, has shown conclusively that 
the gulf-weed is а true floating form, reproducing itself indefinitely by the 
vegetative elongation of the thallus combined with a gradual disintegration 
of the older portions. 
In discussing the probable origin of the gulf-weed, Bürgesen considers 
that it has been derived, probably at a remote period of time, from detached 
specimens of a saxicolous Sargassum and that the change in habitat has 
‘induced an appearance very distinct from the original attached form." 
He considers the most probable ancestors of Sargassum natans, L., as 
either S. vulgare, C. Ag, or S. silipendula, C. Ag. Similarly, S. fluitans, 
Bürg., is probably derived from S. //ystriv, J. Ag. (Borgesen, 1914, р. 222, 
Pheophycer). It is interesting to see how far the change in morphology, 
in such a transition, is stmilar to those we should expect from an examination 
of the detached Fucoids. 
Fig. 17 А & B, which are taken from Borgesen’s paper on Sargassum (19145, 
show the morphology of the attached Surgassum rulgare and its vegetative 
offspring in the floating condition, S. natans. The physical conditions 
under which the floating gulf*weed lives are essentially identical with those 
operating upon the loose-lying Fuci with one important exception. Thus 
we have a uniformity in concentration, both in time and space, which induces 
(a) abortion of the sexual organs and (b) lack of spirality or contortion. 
Secondly, although the Sargasso weed lives in the great ocean-currents, 
which would seem to give it access to unlimited supplies of nutrient. salts, 
in reality, as the plant floats and so moves with the water, it will suffer 
from a shortage of nutrient salts, because it can only lay under contribution 
the volume of water which it oceupies, while an attached plant may absorb 
salts from much larger volumes of water. Hence the attenuation of the 
thallus, which is the most characteristic morphological peculiarity of the Sar- 
gasso weed, is probably directly induced by its floating habit. In the case of 
the loose-lying Fucoids, we saw that the other factor in their dwarf habit 
—the shortening of the thallus—was probably due to a curtailment of 
photosynthesis owing to low illumination and shortage of СО; but the 
Sargasso weed suffers from neither of these disadvantages. It floats on 
the surface and so has abundant light, while the rich fauna, whieh takes 
refuge among its branches, must afford an ample supply of CO, Hence we 
do not get shortening combined with the attenuation of the thallus in Sar- 
gasso weed ; if anything, the foliose fronds are longer in the floating variety 
than in the attached plant. The last morphological peculiarity of the floating 
Sargassum—the absence of eryptostomata upon its thallus—is analogous to 
the absence of eryptostomata in the ecad filiformis series of the loose-lying 
forms of Fucus vesiculosus, but we do not understand the significance of 
these organs. | 
We see, therefore, that all the morphological peculiarities of the floating 
