THE ALG.E 185 



TYPE XIV. ECTOCAEPUS SILICULOSUS 



1. STRUCTURE 



The genus Ectocarpus, various species of which are 

 extremely common on our shores, includes some of the 

 simplest forms of the Phaeophyceae. The thallus is 

 filamentous, and repeatedly branched. It consists of 

 two parts ; a creeping primary portion by which the 

 plant is attached to the substratum (usually one of the 

 larger seaweeds), and a tuft of branched threads, arising 

 from the creeping part and waving freely in the water, 

 often reaching a length of several inches. Throughout 

 the plant the filaments usually remain one cell thick, 

 though in a few cases longitudinal divisions occur. 

 Each cell contains a single nucleus and a varying number 

 of plastids, to the pigment contained in which the brown 

 colour of the whole plant is due. 



The mode of growth of the free filaments is peculiar. 

 Instead of having an apical growing-point, each branch 

 grows chiefly near its base. In this part short meri- 

 stematic cells are found, which multiply by transverse 

 division, while the more apical part of the branch 

 consists of long, full-grown cells, which have ceased to 

 divide (see Fig. 78). These intercalary growing-points, 

 as they are called, are characteristic of this group of 

 plants, though not by any means constant among 

 them. 



2. EEPRODUCTION 



The reproductive organs are of two kinds, which are 

 distinguished as unilocular and plurilocular sporangia. 

 They are usually borne on distinct plants, but sometimes 



