IV. 
INTRODUCTION. 
7 
tivc powers, developed in various chemical solutions or in the waters of mineral 
springs. All the Algse however which are found in such localities are not species 
of Hygrocrocis , for several Oscillatorice and Calothrices occur in thermal waters. 
Species of the former genus are found even in the boiling waters of the Icelandic 
Geysers. Of the latter, one species at least, Calothrix nivea , is very common in 
hot sulphur springs, and I observed it in great plenty in the streams running from 
the inflammable springs at Niagara. 
But on whatever substance the Alga may feed, it is rarely obtained through the 
intervention of a root. Dissolved in the water that bathes the whole frond, the food 
is imbibed equally through all the cells of the surface, and passes from cell to cell 
toward those parts that are more actively assimilating, or growing more rapidly. The 
root, where such an organ exists, is a mere holdfast, intended to keep the plant fixed 
to a base, and prevent its being driven about by the action of the waves. It is ordina- 
rily a simple disc, or conical expansion of the base of the stem, strongly applied and 
firmly adhering to the substance on which the Alga grows. This is the usual form 
among all the smaller growing kinds. Where, however, as in the gigantic Oar-weeds 
or Laminarice , the frond attains a large size, offering a proportionate resistance to 
the waves, the central disc is strengthened by lateral holdfasts or discs formed at 
the bases of side roots emitted by the lower part of the stem ; just as the tropical 
Screw-pine (Pandanus) puts out cables and shrouds to enable its slender stem to 
support the weight of the growing head of branches. The branching roots of the 
Laminaria , then, are merely Fucus-di scs become compound : instead of the conical 
base of a Fucus , formed of a single disc, there is a conical base formed of a number of 
such discs disposed in a circle. In some few instances, as in Macrocystis , the grasp- 
ing fibres of the root develope more extensively, and form a matted stratum of con- 
siderable extent, from which many stems spring up. This is a further modification 
of the same idea, a further extension of the base of the cone. 
In all these cases the roots extend over flat surfaces, to which they adhere by a 
series of discs. They shoiv no tendency to penetrate like the branching roots of per- 
fect plants. The only instances of such penetrating roots among the Algae with 
which I am acquainted, occur in certain genera of Siphonece and in the Caiderpece , tro- 
pical and sub-tropical forms, of which there are numerous examples on the shores 
of the Florida Keys. These plants grow either on sandy shores or among coral, 
into which their widely extended fibrous roots often penetrate for a considerable dis- 
tance, branching in all directions, and forming a compact cushion in the sand, 
reminding one strongly of the much divided roots of sea-shore grasses that bind 
together the loose sands of our dunes. But neither in these cases do the roots 
appear to differ from the nature of holdfasts, and their ramification and extension 
through the sand is probably owing to the unstable nature of such a soil. It is not 
in search of nourishment, but in search of stability, that the fibres of their roots are 
put forth, like so many tendrils. We shall have more to speak of these roots in 
the proper place, and shall now proceed to notice some of the forms exhibited by 
