Div. i THALLOPHYTA 409 



division of the contents, escape by an opening at the summit. Each has a single 

 cilium and contains two chloroplasts. After swarming the spore surrounds itself 

 with a wall and grows into one of the balloon-shaped plants. Sexual reproduction 

 is not known ( 2 ). 



CLASS X 

 Phaeophyeeae (Brown Algae) ( J > " 39 ' 47 ) 



The Phaeophyeeae, like the Chlorophyceae, can be derived from 

 Flagellata. They attain a higher grade of organisation in their 

 vegetative organs than do the Green Algae. 



With the exception of a very few fresh- water species, the 

 Phaeophyeeae are^ only found in salt water. They attain their 

 highest development in the colder waters of the ocean. They 

 show great diversity in the form and structure of their vegetative 

 body. The simplest representatives of this class (e.g. the genus 

 Ectocarpus) have a filamentous thallus consisting of a branched or 

 unbranched row of simple cells. Some Phaeophyeeae, again, have a 

 cylindrical, copiously-branched, multicellular thallus (e.g. Cladostephus), 

 whose main axes are thickly beset with short multicellular branches 

 (Fie. 89) : while in other cases the multicellular thallus is ribbon-shaped 



\ O / ' * 



and dichotomously branched (e.g. Dictyota, Fig. 83). Growth in length 

 in both of these forms ensues from the division of a large apical cell 

 (Figs. 89 and 90). Other species, again, are characterised by disc- 

 shaped or globose thalli. 



The Laminariaceae and Fucaceae include the most highly developed 

 forms of the Phaeophyeeae. 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 digitata and L. Cloustoni (Fig. 351), a zone at the base of 

 the palmately-divided, leaf-like expansion of the thallus retains its meristematic 

 character, and by its intercalary growth produces in autumn and winter a new 

 lamina on the perennial stalk. 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 characteristic of the 

 Laminarias ; L. saccharina (North Sea), for instance, has an undivided but 

 annually renewed lamina, frequently 3 m. long, and a stalk more than 1 cm. thick. 



The greatest dimensions attained by any of the Phaeophyeeae are exhibited by 

 certain of the Antarctic Laminariaceae. Of these, Macrocystis pyrifera(^\g. 350) 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 (&), is at first dichotomously branched. 

 Single shoots of the thallus grow to the surface of the water, and floating there attain 

 a great length ; they bear on one side long flat lobes divided at their free ends, 

 and having at the base of each a large swimming bladder. In the Antarctic 

 SKOTTSBERG measured examples 70 m. long, while FRYE, RIGG, and CRANDALL 



