PH^OSPORE^. Ill 



single row of superposed cells in some species oi Ectocarpus ; 

 while in Laminaria, the largest of our brown seaweeds, 

 popularly known as " devil's aprons," the thallus is differ- 

 entiated into portions resembling in appearance, but not in 

 function, root, stem, and leaf. In the stems of Macrocysiis 

 and Nereocystis there is a considerable amount of differenti- 

 ation exhibited ; elongated cells called trumpet-hyphce occur, 

 having the transverse septa and also lateral portions of the 

 wall perforated and forming true sieve-plates, with a callus 

 formation similar in structure to the sieve-tubes met with in 

 the bast of some Phanerogams. In some species increase in 

 size depends on the division of an apical cell ; in others 

 that form crust-like expansions, increase in size takes place 

 by the simultaneous division of the tips of several con- 

 tiguous filaments, which collectively constitute a peripheral 

 generating zone. A third mode of growth, known as inter- 

 calary, is illustrated by the genus Laminaria, where the point 

 of growth is situated at the junction of " stem " and " leaf" ; 

 the new "leaf" originates at the apex of the stem, and 

 growing by addition to the base, gradually pushes off the 

 old " leaf." In Scytosiphon, Chorda, and some other genera 

 where the thallus is unbranched and the " stem " absent, 

 the point of growth is near the base of the thallus. 



Protoplasmic continuity is very clearly shown, especially 

 in the larger species. 



When young, the thallus of many species is covered with 

 very slender hair-like outgrowths from the superficial cells, 

 which usually disappear with age. 



The olive-brown colour is due to the presence of a 

 pigment called phycophmin. 



In many of the larger species the thallus is enabled to 

 float on or near the surface of the water by means of air- 



