366 



BOTANY 



The Laminariaecae and Fucaceae include the most highly developed 

 forms of the Phaeophyceae. To the first family belongs the genus 

 Laminaria found in the seas of northern latitudes. The large- 

 stalked thallus of the Laminarias resembles an immense leaf ; it is 

 attached to the substratum by means of branched, root-like holdfasts, 

 developed from the base of the stalk. 



In Laminaria diyilata and L. Clonstoni (Fig. 301), a zone at the base of 



Fio. 300. Macrocystis'pyrijem, Ag. ; a, 

 younger 6, older thallus. (A ft IT 

 SKOTTSBERO. ^, nat. size.) 







the palmately divided, leaf -like expansion 

 of the thallus retains its meristematic 

 character, and by its intercalary growth 

 produces in autumn and winter a succes- 

 sion of new laminae. The older lamina 

 becomes pushed up and gradually dies, while 

 a new one takes its place and becomes in turn 

 palmately divided by longitudinal slits. The 

 large size of their thalli is also charact< ! Nti<- 

 of the Laminarias ; L. saccharina (North Sea), 

 for instance, is frequently 3 m. long and the 

 stalk more than 1 cm. thick. 



The greatest dimensions attained by any 

 of the Phaeophyceae are exhibited by certain 

 of the Antarctic Laminariaceae. Of these, 

 Macrocystis pyrifera (Fig. 300) is noted for its 

 gigantic size ; the thallus grows attached to 

 the sea-bottom at a depth of 2-25 m., and, according to SKOTTSBERG ( M ), is at first 

 dichotomously branched. Single shoots of the thallus grow to the surface of the 

 water, and floating there attain a length of 70 m. ; they bear on one side long flat 

 lobes divided at their free ends, and having at the base of each a large swimming 

 bladder. Other noticeable forms, on account of their tree-like character, an- the 

 Antarctic species of Lessonia, in which the main axis is as thick as a man's arm ; 

 from it are given off lateral branches with hanging leaf-like segments. The plant 

 attains a height of several metres, and has a tree-like habit of growth. 



FIG. 301. Laminaria 



Sea. (Reduced to J.) 



