[Chap. XLVII THE ALGAE 631 



of the kelps are perennial, and the stalk increases in diameter bv the 

 formation of new rings of peripheral cells. 



In the upper littoral zone along our coasts there grows in great 

 abundance a branched, filamentous brown alga, known as Ectocarpiis. 

 It is practically world-wide in distribution. Along the Atlantic coast it 

 is common as an epiphyte on the rockweeds (Fiiciis and Ascophyllum), 

 and less common along the Pacific coast on certain kelps. Motile spores 

 produced from special cells may germinate and from them new plants 

 develop. Other filaments produce flagellate gametes which upon fusion 

 form zygotes. The germination of the zygote results in a plant that is 

 able to produce only motile spores. In general, one cannot distinguish 

 the filaments that produce motile spores from those that produce gametes 

 until the reproductive structures actually appear. There occurs then in 

 Ectocarpiis an alternation of asexual and sexual phases similar to those of 

 seed plants, ferns, and mosses. Reduction division takes place during the 

 formation of the motile spores. 



Kelps. The body of a kelp consists of a holdfast, a stipe, and a very 

 large blade. Kelps grow only in the colder waters of the oceans; they 

 are absent from the warm waters of tropical and semitropical regions. 

 Most species of kelp grow below low tide and are thus permanently in 

 water. The life cycle of the kelps may be indicated by a study of 

 Laminaria, sometimes called devil's apron, found along both coasts of 

 this continent. On the surface of the blade occur patches of sporangia, 

 from which motile spores escape (Fig. 289). From these spores minute 

 gamete-bearing filaments develop. These tiny plants may be few-celled 

 or many-celled, and each one is unisexual, producing either only male 

 gametes or only female gametes. The union of two gametes results in a 

 zygote, from which the large plant we know as the kelp de\'elops. We 

 thus have a pair of microscopic gametophytes ( gamete-bearing plants ) 

 alternating with a large sporophyte ( spore-bearing plant ) . 



The large blade-like kelp is the diploid plant. Reduction division 

 occurs in it during the formation of the motile spores, which are the fore- 

 runners of the small unisexual gametophytes. 



It is interesting to recall that in nearly all green algae the conspicuous 

 plant is the one bearing gametes. In Laminaria, and a few other algae as 

 well, the gamete-producing plant is microscopic; the spore-bearing plant 

 is the conspicuous structure, as it is in the seed plants. Moreover, the 

 conspicuous phase of Laminaria is diploid, and the microscopic game- 

 tophytes are monoploid. The conspicuous phase of green algae usually 



