DEVELOPMENT OF PLANTS 



187 



bladders which buoy it up in the water ; this feature accounts 

 for its popular name of bladder wrack. A cross section of the 

 stem shows that the tissues of these plants have attained a con- 

 siderable differentiation (Fig. 122, A) as is attested by a rudi- 

 mentary epidermal, cortical and central region, the latter often 

 containing well marked sieve tubes. 



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 Fuciis (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 

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



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. 



