208 



SEXUAL REPRODUCTION OF FUCUS 



The most characteristic feature of these plants, however, and 

 the one separating them sharply from the Laminarias appears 

 in their method of reproduction. None of the Fucaceae develop 

 zoospores, although an asexual reproduction may be effected by 

 the detachment of small branches from the plants. Sexual re- 

 production is a step in advance of any of the preceding types 

 of the brown algae, in that the female gamete becomes still 

 larger and loses entirely the power of motion. The reproductive 

 organs, or gametangia, are developed in specialized branches or 

 enlarged tips of the thallus. In Fucus (Fig. 121, g), these organs 

 are contained in small pits or cavities that appear as minute 

 points, or as dots when the enlarged tip of a branch is held up 

 to the light. A magnified section taken through such a branch 

 shows the nature of the cavities (Fig. 122, B, C). In some 

 species, the male and female gametangia are found in the same 

 cavity, or they may occur separately and on different plants. 

 The male gametes are developed in enormous numbers in numer- 

 ous little sacs, or antheridia, borne on branching filaments of cells 



Fig. 123. One of the branching filaments from Fig. 122, B, greatly en- 

 larged. Some of the antheridia, an, have discharged the male gametes, which 

 are still retained in the inner wall of the antheridium, as at b. At a this wall 

 is ruptured, freeing the gametes. 



which grow out from the sides of the cavities (Fig. 123). The 

 female gametes are produced in larger sacs or oogonia, which 

 are each supported on a single cell and associated with hair-like 

 chains of cells, paraphyses (Fig. 124, A). Usually eight gametes 

 are developed in each oogonium. The gametes when first dis- 

 charged from the antheridia and oogonia are enclosed in a delicate 



