278 BOTANY: PRINCIPLES AND PROBLEMS 



celled, sperm-producing filaments. The oogonium is covered 

 with a wall or envelope of spirally wound cells growing up from 

 the tissue below it, and produces a single large egg. After fer- 

 tilization the envelope hardens, forming a nut-like spore case 

 around the oospore. The absence of anything suggesting an 

 alternation of generations indicates that these plants should be 

 placed among the thallophytes, but they are clearly distinct 

 from any other members of the division. 



Phaeophyceae or Brown Algae. — These plants may be dis- 

 tinguished from other algae by their characteristic brown color, 

 due to one or more brown pigments associated with chlorophyll, 

 and by certain structural characters. The Phaeophyceae are 

 the largest and rankest of all algae and display the highest degree 

 of bodily differentiation. They are found almost exclusively in 

 salt water and are best developed in the cooler seas. Thriving 

 most commonly in shallow water and the zone between tide 

 marks, they are subjected to the buffeting of the waves and may 

 be exposed to the air for several hours a day. These plants 

 probably represent an entirely independent line of evolution 

 from the green algae and seem to have led to no higher types. 

 Two orders are recognized among them, the Phaeosporales and 

 the Fucales. 



1. Phaeosporales or Kelps and Their Allies. — In this order 

 occur the kelps (Laminaria) common in the north Atlantic and 

 elsewhere, together with many other large algae such as the giant 

 kelp (Macrocystis) , the sea otter's cabbage (Nereocystis), the 

 sea palm (Postelsia), and others smaller in size. They are all 

 isogamous and in most cases produce zoospores. 



Ectocarpus (Fig. 157), one of the best known genera, is a 

 rather small, filamentous plant. As in the algae previously 

 studied, the zoospores are here produced in sporangia which 

 are modified single cells. The gametes, however, are developed 

 in large multicellular structures (plurilocular sporangia) which 

 begin to show a resemblance to the highly developed sexual 

 organs characteristic of the bryophytes. Each of the many 

 small cells into which the contents of this structure is divided 

 forms one or two gametes which fuse in pairs to produce zygo- 

 spores. In,stances have been observed in which these gametes 

 germinate directly into a new plant, and thus function essentially 

 like zoospores, which they also resemble structurally. Gametes 

 of different size sometimes unite, thus indicating the beginning of 



