The Algae - Phaeophycae

Phylum Thallophyta - The Algae - Phaeophycae
The Phaeophyceae are Algae in which an aditional 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 zoo­spores, 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 mayor 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 sporo­phyte 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.
The Phaeophyceae are divided into a number of orders of which we shall
consider examples from five :-

1. Ectocarpales (e.g., Ectocarpus). 
2. Cutleriales (e.g., Cutleria). 
3. Dictyotales (e.g., Dictyota). 
4. Laminariales (e.g., Laminaria). 
5. Fucales (e.g., Fucus). 

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