. A textbook of botany for colleges and universities ... Botany. 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 dissim
. A textbook of botany for colleges and universities ... Botany. 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- gassiim (gulfweed). 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 aquatic form or of asexual spores of any kind is hard to understand, branch tips, and the air Fig. 127. — Fucus: showing the dichoto- mous thall
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