THALLOPHYTES 



49 



blades or on special blades, and the gametes which they produce were 

 formerly mistaken for zoospores. It seems probable that zoospores have 

 been eliminated from the life history of the kelps, as in the Fucales, and 

 that the only spores are zygospores. 



Cutleriaceae. The body in this family is a broad, flat, forking thallus, and 

 zoospores are formed as in other Phaeosporales ; but there is a differentiation 

 of gametes that deserves attention. Gametangia of two kinds are produced, simi- 

 lar in appearance, but dissimilar in gametes. One kind of gametangium produces 

 fewer and larger gametes, the other more numerous and smaller gametes, and 

 both kinds are ciliated and set free. In such a case, the two kinds of gametangia 

 may be regarded as multicellular oogonia and antheridia, the fusion as fertilization, 

 and the product as an oospore. Cutleriaceae, therefore, may be taken to represent 

 a transition from Phaeosporales to Fucales. 



(b) Fucales 



General character. This relatively small and specialized group of 

 brown algae is characterized by the absence of zoospores and the pres- 

 ence of well-developed heterogamy. The common representatives are 

 Fucus (rockweed) and Sar- 

 gf.ssum (gulf weed). 



Fucus. The body of these 

 exceedingly common forms is a 

 flat thallus which forks re- 

 peatedly (fig. 127), a type of 

 branching called dichotomous . 

 It grows by means of an apical 

 cell, which soon becomes placed 

 at the bottom of a notch by the 

 more rapid growth of the two 

 branches. . The body is attached 

 to its support by a basal disk, 

 and is made buoyant by air 

 bladders or floats that are in- 

 flated intercellular spaces. There 

 is also a distinct differentiation 

 of tissues into the more compact 

 cortex and the looser medulla. 



The absence of zoospores in an 



FIG. 127. Fucus: showing the dichoto- 



aquatic form or of asexual spores mous tha iius, the region of sex organs at the 

 of any kind is hard to understand, branch tips, and the air bladders, 

 C. B. & C. BOTANY 4 



