XX INTRODUCTION. 



Alg3D appear to be as much influenced by the soil in which 

 they grow as other plants are, and a large number of those 

 that are parasitical confine themselves to particular species. 

 This selection of habitat would seem to prove that the 

 root is not so sluggish an organ as it has been supposed to 

 be. It does not, however, present much modification, and 

 rarely attains a large size. The usual form is that of a 

 hard, callous disk ; sometimes this is accompanied by fleshy 

 fibres ; and occasionally, but rarely, the root consists of an 

 extensive creeping mass of fibres. This latter form is 

 most remarbable in the genus Caiderpa, the species of 

 which grow on sand, and consequently require the support 

 of an extensively ramified, penetrating and compact root. 

 Some species, which, under ordinary circumstances, are 

 attached by roots, occasionally dispense with them, and 

 continue to flourish independently of them. Of these the 

 most remarkable are Sargassum hacciferum and vulgare (.?), 

 which, under the Spanish name " Sargasso," or the English 

 " Gulf-weed," have forced themselves on the notice of all 

 voyagers who have crossed the Atlantic since the time of 

 Columbus. The vast fields of sea-weed which were met 

 with by " the adventurous Genoese" and his early follow- 

 ers, which made the ocean appear like a meadow, and sen- 

 sibly impeded the course of their small vessels, consisted of 

 these species. According to Humboldt there are two 

 principal banks ; one, the largest, extending from the 25th 

 to the 36th degree of north latitude, and a little west of the 

 meridian of Fayal, one of the Azores ; the other, w^hich is 

 much smaller, a short way west of the Bahamas, and be- 

 tween the 22nd and 26th degrees of latitude. These 

 localities of the banks, however, can but be considered as 

 approximations, for with plants that float about wherever 

 the winds and currents drive them no very certain station 

 can obtain. Vessels returning from the Cape of Good 



