BROWN SEAWEEDS 79 



him frequently enough by becoming tangled about the 

 oars of his boat. This weed is found quite 40 feet in 

 length; it is seldom more than I inch in diameter. But 

 these species are mere dwarfs in comparison with some 

 of the great Kelps of the Pacific and the Southern 

 Seas — for instance, -Macrocystis has an exceedingly 

 slender stem, about | inch in diameter, which attains 

 a length of hundreds of feet; this is surmounted by an 

 air-vessel, about 1 inch in diameter, to which a number 

 of ribbon-like leaves are attached. There are nearly 

 one hundred species in_^the family Laminariacese; they 

 do not thrive in the tropics, but are numerous on the 

 coasts of Arctic and temperate seas. 



It is remarkable that in a family of Seaweeds in which 

 there is such a decided and even complex development 

 of the vegetative thallus the mode of propagation is of 

 a very simple order. Sexual cells have not been ob- 

 served among the Laminariacese; the plants are known 

 to liberate asexual zoospores which give rise to new 

 plants. But we have yet much to learn concerning the 

 life-cycle of the members of this and other families of 

 Seaweeds. 



Let us turn our attention to the genus Ectocarpus, of 

 which there are several British species. Here we find 

 some of the simplest Brown Algae. They are well dis- 

 tributed throughout the coastal regions of the world, 

 and are recorded as being abundant in the North 

 Atlantic. Ectocarpus siliculosus forms tufts of slender, 

 much-branched filaments one cell thick; the colour is 

 brownish or olive-green. This repeatedly divided fila- 

 mentous thallus has a creeping base by which the plant 

 attaches itself to a substratum; it is usually found 



