68 



PLANT MORPHOLOGY 



■1^ 



M. 



Ectocarpus displays a primitive type of alternation of generations and 

 one that is not well established. Although all the plants of a species are 

 alike vegetatively, some are gametophytes and some are sporophytes. 

 The gametophytes, producing gametangia, are haploid. The zygotes 

 give rise to sporophytes, which are diploid. These produce two kinds of 

 sporangia. One is mvilticellular and looks like a gametangium but gives 



rise to diploid zoospores that develop into 

 other sporophytes. The other is the uni- 

 cellular sporangium already described. 

 The division of the nucleus in the young 

 unicellular sporangium is reductional, 

 and so the zoospores that it produces 

 are haploid. These haploid zoospores 

 always give rise to gametophytes. Some- 

 times gametes, without pairing and 

 fusing, develop directly into other hap- 

 loid plants. It is apparent that much 

 variation occurs in the behavior of the 

 spores and gametes. 



Pylaiella. This alga resembles Ecto- 

 carpus in its habitat and general struc- 

 ture. It differs chiefly in that the fila- 

 ments are usually only slightly branched 

 and any cell may become a sporangium 

 or gametangium (Fig. 55). Conse- 

 quently the reproductive organs are in- 

 tercalary in position and usually appear 

 in a linear series. They have the same 

 structure as those of Ectocarpiis, the 

 sporangia being unicellular and the gam- 

 etangia multicellular. Sometimes mul- 

 ticellular sporangia are produced on the 

 plants bearing unicellular ones. Although Pylaiella is essentially 

 isogamous, one of the pairing gametes is slightly larger than the other. 

 An alternation of generations is seen also in this genus, the gamete- 

 producing plants being haploid and the spore-producing plants diploid. 

 The reduction of chromosomes occurs in the young unicellular sporan- 

 gium. 



Summary. The thallus of the Ectocarpales is usually composed either 

 of freely branching filaments or wholly or in part of a plate-like or solid 

 body composed of interlacing filaments. In some forms the thallus is 

 parenchymatous. Vegetative growth is mainly intercalary, often being 

 confined to basal portions of the branches. Reproduction occurs typi- 



■C 



■% 



1» 



A B 



Fig. 55. A row of sporangia (A) and 

 gametangia (B) oi Pylaiella, X500. 



