MODE OF GROWTH OF SEA- WEEDS. 41 



shown (Chap, iv.) how strong the contrast is, between the func 

 tions of the true roots of Flowering-plants, and the root-like 

 organs of the Algse. 



42. The higher Algae sometimes attain a prodigious extent of 

 development, forming vast submarine forests of the most luxuriant 

 vegetation. Thus the Chorda filum, a species common in the 

 North Sea, is frequently found of the length of 30 or 40 feet ; 

 and in the neighbourhood of the Orkneys, it forms meadows, 

 through which a boat forces its way with difficulty. It grows in 

 the form of a long and even cord (whence its name), about the 

 size of a quill, attached at one end to the bottom or shore, and 

 the rest supported by the water. This is nothing, however, to 

 the prodigious extent of the Macrocystis pyrifera, which is reported 

 to be from 500 to 1500 feet in length, the long and narrow fronds 

 having an air-vesicle at the base of each, the stem not being thicker 

 than the finger, and its upper branches as slender as common 

 pack-thread. Another tropical species attains the length of 25 

 or 30 feet, with a trunk as thick as a man's thigh. Sometimes 

 these stems are solid, and sometimes hollow ; the tubular stem of 

 one species of Laminaria, found near the Cape of Good Hope, 

 has been used by the natives as a trumpet, when dried. An- 

 other species furnishes the natives of some parts of Australia, 

 with a large proportion of their instruments, vessels, and even of 

 their food. 



43. The marine Algaa differ much in their habits. Some 

 species grow altogether beneath the water, attaching themselves 

 below the lowest tide-level. Others fix themselves where their 

 fronds may float on the surface, and may be exposed in some 

 degree to the direct influence of the air. Others again frequent 

 a height, at which they are left dry at every retreating tide ; and 

 some are found in situations, in which they are scarcely ever 

 covered by water, thus approaching in habits, and in character 

 also, to the Lichens. Although most attach themselves to rocks 

 or other solid masses, frequenting the shores or shallows rather 

 than the open sea, there are some exceptions, among which one 

 of the most remarkable is the Sargasso or Gulf Weed, which 

 floats on the surface of the ocean, in the Gulf of Mexico, and 

 in the current which sets from this towards the north. Immense 



