CHAPTER V 



THE ALGAE : PHAEOPHYCEAE, BACILLARIOPHYCEAE 



AND RHODOPHYCEAE 



PHAEOPHYCEAE 



The Phaeophyceae are Algae in which an additional pigment, fucoxanthin, 

 is developed, more or less completely masking the chlorophyll which is also 

 present in the cells. Unicellular types occur, and there are a number of 

 genera in which the majority of the species are filamentous, but the bulk of 

 the species included in this group have a more complex thallus built up 

 of either cellular tissue or of a number of central filaments bound together 

 by mucilage, each of which gives off lateral branches in such numbers that 

 they form a tissue surrounding the central filaments. In such types the 

 central filaments are termed the medulla, while the surrounding tissue 

 is spoken of as the cortex. In the most advanced types the cells of the 

 medulla become modified into specialized conducting elements, which assist 

 in the transference of the food material from one part of the plant to another. 

 We find, then, in the higher Phaeophyceae much greater morphological 

 differentiation than is met with in the Chlorophyceae. 



Sexual reproduction by means of isogametes is the method most common 

 among the lower orders, while in the higher orders reproduction may be 

 either isogamous or oogamous. Asexual reproduction is by means of zoo- 

 spores, which differ from those of the Chlorophyceae in being pear-shaped 

 with two laterally placed flagella. 



Many of the Phaeophyceae exhibit alternation of generations, that is to 

 say, there is a definite and immutable alternation of sexual and asexual 

 individuals. The zygote develops into a plant which may or may not resemble 

 the parent and on which only asexual reproductive bodies are borne. These 

 on germination produce a plant, not necessarily like its parent, but like the 

 " grandparent," on which sexual reproductive bodies are developed. The 

 plant which produces the gametes is termed the gametophyte, and that 

 which develops the asexual spores is called the sporophyte. In alternation 

 of generations therefore there is an alternation of gametophyte and sporo- 

 phyte generations. This obligatory alternation of generations is extremely 

 important, and although the Phaeophyceae are the first group in which we 

 meet it as a general characteristic, we shall find that in all higher plants it 

 invariably occurs, though it may not be easy to recognize it at first sight, 

 owing to secondary modifications which have taken place in the course of 

 evolution. 



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