202 SIMPLER BROWN ALGAE 



probable that the brown algae have been derived from green 

 algae but there is no satisfactory evidence of a series of unicellu- 

 lar forms leading up to the rather complex types that are repre- 

 sentative of the simplest of the Phaeophyceae. The majority 

 of these lower forms have already reached the filamentous stage 

 and many of the higher genera exhibit a differentiation of the 

 plant body suggestive of the higher plants, as for example, a 

 definite axis with branches and leaf-like outgrowths and root- 

 like organs that anchor the plants to the substratum. Likewise, 

 the tissues of these higher types often reveal many of the features 

 already noted in the epidermis, cortex and central region of 

 terrestrial plants. The life history of the brown algae indicates 

 that they have undergone the same evolution as the Chloro- 

 phyceae. For purposes of comparison we will consider only 

 two groups of the brown algae. 



(a) The Simpler Broivn Algae.- — Ectocarpits may be taken as 

 an example of this group. It is a filamentous branching form 

 that is almost universally distributed along the sea shore (Fig. 

 119). The zoospores are developed in certain cells of the upper 

 branches. It is important to note in the brown algae, that 

 the reproductive cells are generally confined to special parts 

 of the plant and not promiscuously developed as in the green 

 algae. This marks a decided advance in the evolution of the 

 plant. The tissues are becoming more specialized and the work 

 performed by the plant is apportioned to special cells and organs. 

 The zoospores have essentially the same structure and mode of 

 growing into new plants as noted in Ulothrix. The cilia, how- 

 ever, are laterally attached and this is a characteristic feature 

 of the zoospores of the Phaeophyceae generally (Fig. 119, A, g). 



The sexual reproduction of Ectocarpus exhibits some of the 

 most instructive variations to be found in all the algae. The 

 gametes are formed in gametangia that become divided into 

 great numbers of very small cubical cells, each one of which 

 produces a single gamete (Fig. 119, B). Thus the origin of 

 the gametes is more specialized than in the case of the green 

 algae where numerous g metes were formed in a single cell. 

 You are to remember this feature of the gametangiu be ause 



