130 



BOTANY 



plant-body in Cutleria (Fig. 100) is a flat, dichotomously branched 

 thallus, growing by intercalary divisions, much as in the Ectocar- 

 paceae. Zoospores, like those of the other Phaeosporeae, are formed, 

 and the gametangia are very similar, but are of two kinds, producing 

 gametes of very unequal size, although both male and female 

 gametes are ciliated. 



con 



Order Cyclosporeae 



The Fucaceae comprise the most specialized, and next to the Lami- 

 nariaceae, the largest of the Phaeophyceee. They are distinguished 



by having the egg-cell 



A very much larger than the 



spermatozoids, and desti- 

 tute of cilia. 



The best known of these 

 are the Eockweeds of the 

 genus Fucus, which are 

 widely distributed through 

 the colder waters of the 

 northern hemisphere. An- 

 other familiar form on our 

 northern Atlantic coast 

 is Ascophyllum nodosum, 

 whose slender rubberlike 

 branches, with large air- 

 vesicles at intervals, hang 

 down from the rocks at 

 low tide. In the warmer 

 seas the Fucaceae are rep- 

 resented by numerous 

 genera, the largest being 

 Sargassum. S. bacciferum, 

 the Gulfweed, is common 

 floating in the Gulf 

 Stream. 



The species of Fucus 

 (Fig. 101, A) have a dicho- 

 tomously branched thal- 

 lus, tapering below into 

 a stout stalk, which is attached by a disk. The branches are either 

 nearly cylindrical or, in some species, flattened, with a prominent 

 midrib and thin, winged margins. There may be seen, in some 

 species, little tufts of fine hairs scattered over the thallus, and these 

 are found to project from small pits (Cryptostomata), within which 



FIG. 101. A, Fucus vesiculosus. JB, C, Halidrys 

 Osmundacea; v, vesicles; con, conceptacles. 



