160 FUNDAMENTALS OF CYTOLOGY 



plant. Cladophora glomerata is also reported to be a diplont, but it 

 differs from C odium in having zoospores: these occur throughout the 

 year, while gametes are formed only in the spring. 



The hrown algae also exhibit life cycles of the three main types 

 described above. Ectocarpus virescens is a haplont. The plant body 

 produces biciliate gametes which fuse in pairs, and the resulting zygotes 

 undergo meiosis as they germinate to form new plants. Other types of 

 cj^cle are shown by certain other species of the genus. Evidentl}^ the 

 same species may differ in this respect in the North Atlantic Ocean and 

 the Mediterranean Sea. 



Dicfyota dichotoma is a diplohaplont. There are three kinds of plant 

 .similar in external appearance. Monoploid female plants produce 

 large nonmotile eggs which are liberated into the sea water. These 

 are fertilized by small uniciliate sperms from monoploid male plants. 

 The zygotes germinate in a few hours and develop into diploid plants 

 producing tetraspores. These tetraspores, which are products of 

 meiotic divisions, develop into new sexual plants. In Lami7iaria the 

 gametophytic phase is very minute and consists of onl}^ a few cells. The 

 very large body (kelp) is the sporophytic phase. 



Fucus and species of certain other genera are diplonts. Here the 

 plant bod}^ is diploid and produces large motile eggs and very small 

 laterally biciliate sperms. Some species are dioecious. The zygotes 

 very soon germinate and develop into new diploid bodies. Since meiosis 

 occurs in the divisions of the gametogenous cells in the sex organs, there 

 are no monoploid vegetative cells in the cj^cle. There are no zoospores. 

 It is thought that this form of cycle, which is very rare in plants, may 

 have arisen through the assumption of gametic functions by spores. 

 This should not be surprising in view of the fact that the spores in many 

 plant groups show a differentiation into two types correlated with the 

 sex of the plants they produce. It is of further interest to note that in 

 brown algae (e.g., Cutleria) having zoospores these cells and the male 

 gametes are both laterally biciliate. 



Of the several cycle types observed among the red algae only two 

 will be mentioned here. In Polysiphonia violacea there are, as in the 

 brown alga Dictyota dichotoma, morphologically similar plants of three 

 kinds: monoploid females, monoploid males, and diploid tetraspore- 

 bearing plants. The female gamete is at the base of a flask-shaped cell 

 with a long hair-like extension, the trichogyne. Small nonmotile male 

 gametes (spermatia) from male plants become attached to the trichogynes. 

 A spermatium nucleus enters the trichogyne and moves through it to 

 the female nucleus, with which it unites. The diploid nucleus then 

 divides, and the cell sends out short filaments from the ends of which 

 diploid carpospores are budded off. About the mass of carpospores the 



