144 A TEXTBOOK OF THEORETICAL BOTANY 



direction of the incident light. The lower of the two cells contains few 

 plastids and develops into the holdfast while the rest of the plant grows from 

 the upper cell. As growth continues a few filaments of cells develop at the 

 lower end and form what are called rhizoids, while at the other end the 

 thallus, which is at first rounded, becomes flattened. The plant soon becomes 

 permanently anchored to a rock or similar support by the aid of the rhizoids, 

 from which later the holdfast is developed (Fig. 132). 



In the life-history of Fucus, therefore, we see a highly developed sexual 

 method of reproduction, which is not associated with any kind of asexual 

 one. The life-cycle may therefore be represented by the diagram (Fig. 133). 



Oospore 



Oosphere 



AntheVozoid 



Oogonium 



Antheridium 



Fig. 133. — Life-cycle oi Fucus. 



It is interesting to note that in certain closely allied genera not all the 

 eight oospheres mature, although in every case eight nuclei are produced 

 in the oogonium. In the genus AscophyUum four out of the eight nuclei 

 function, forming the nuclei of four oospheres, while In Pehetia two oospheres 

 are formed, and in Sargassum, the floating Sargasso Weed, only one out of the 

 eight develops into an oosphere. The fact that in every case eight nuclei 

 were produced led to an investigation of their cytology, and it was found that 

 the reduction division occurred in the first divisions of the oogonium and 

 antheridium, so that the mature oosphere and antherozoid in each case is a 

 monoploid structure. Reduction division immediately preceding the forma- 

 tion of the gametes is common in animals, but in plants it usually occurs 

 either in the formation of the asexual spores or after germination of the 

 zygote, and in this respect Fucus is exceptional — a point possibly associated 

 with the absence of any method of asexual reproduction. 



It may occasion surprise that a diploid plant Wkt Fucus should bear gametes, 

 which appears to contradict what has previously been said about the laws of 

 alternation of generations (p. 1 10). The paradox is explained by the suggestion 

 that the initial cell in both oogonium and antheridium is really a sporangium, 

 which proceeds directly to the formation of gametes without any intervening 

 vegetative stage (cf. Ectocarpus, see p. 115). This is not so far-fetched as 



