IV. INTRODUCTION. 7 



tive powers, developed in various chemical solutions or in the waters of mineral 

 springs. All the Alga3 however which are found in such localities are not species 

 of Hygrocrocls, for several Oscillatorice and CaJothrices 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 Niagai'a. 



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 fi'ond, 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-discs 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 tlie root deA'-elope 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 show no tendency to penetrate like the branching roots of per- 

 fect plants. The only instances of such penetrating I'oots among the Algae with 

 whicla I am acquainted, occur in certain genera oi 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 



