366 BOTANY part ii 



agiuous contents, which break up into a number of swarming spermatozoids. The 

 spermatozoids are very small, and have a single nucleus and two cilia inserted on 

 one side. They collect around the receptive spot of the egg-cell, into which one 

 spermatozoid finally penetrates. After the egg-cell has been fertilised by the 

 fusion of its nucleus with that of the spermatozoid, it becomes invested with a 

 wall and converted into a resting oos2Jore. On germination the oospore grows into 

 a filamentous thallus. 



Class X 



Phaeophyceae (Brown Alg-ae) C' -^' ^-) 



The Phaeophyceae, like the Chlorophyceae, can be derived from 

 unicelluhir Flagellata, and in particular from the Chrysomonadinae 

 which possess yellow chromatophores. 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 sj^ecies, the 

 Phaeophyceae are only found in salt water. They attain their 

 highest development in the colder Avaters 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 

 Edocarpus) have a filamentous thallus consisting of a branched or 

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

 cylindrical, copiously branched, multicellular thallus {e.g. Gladostephus), 

 whose main axes are thickly beset with short multicellular branches 

 (Fig. 7) ; while in other cases the multicellular thallus is ribbon-shaped 

 and dichotomously branched (e.g. Dictijota, Fig. 8). Growth in length 

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

 (Figs. 7 and 111). Other species, again, are characterised by disc- 

 shaped or globose thalli. 



The Laminariaceae and Fucaceae include the most highly developed 

 forms of the Phaeophj-ceae. 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 hy means of branched, root-like holdfasts, 

 developed from the base of the stalk. 



In Laminaria digitata and L. Cloustoni (Fig. 290), 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 iierennial stalk. The older lamina becomes jnished \\\) 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. sacchariiuc (North Sea), for instance, has an imdivided, but 

 annually renewed lamina, IVeij^uently 3 m. long and a 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, Ifacrijcijslis pi/rif era {Fig. 2S9) is 

 noted for its gigantic size ; the tliallus grows attaclied to the sea-bottom at a depth 



