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- 
gassum (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 
. f e FIG. 127. Fucus: showing the dichoto- 
aquatic form or of asexual spores mous thalluSj ^ regicn of sex organs at ^ 
of any kind is hard to understand, branch tips, and the air bladders. 
