76 



PLANT MORPHOLOGY 



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attached by a disk-shaped mass of rhizoids (Fig. Q2A). It grows by- 

 means of a row of apical cells extending around the distal margin of each 

 branch. The mature thallus is about eight layers of cells in thickness. 



The diploid sporophytes bear groups of sporangia 

 intermixed with paraphyses. Each sporangivim, lack- 

 ing a stalk cell, gives rise to eight large haploid aplan- 

 ospores (Fig. 625, C). These produce the gameto- 

 phytes, which are either male or female and resemble 

 the sporophytes vegetatively. The antheridia and 

 oogonia are, in general, similar to those of Dictyota 

 (Fig. 62Z)-G). The zygote produces a sporophyte. 



Summary. The thallus of the Dictyotales is flat, 

 plate-like, and erect. It grows by means of a single 

 apical cell or a marginal row of apical cells. Repro- 

 duction occurs by aplanospores, four or sometimes 

 eight being developed in a unicellular sporangium, 

 and by heterogametes. Small biciliate sperms are 

 borne in multicellular antheridia and large nonmotile 

 eggs are borne singly in unicellular oogonia. A 

 distinct alternation of generations is present, the gam- 

 etophyte and sporophyte being similar vegetatively. 



5. Laminariales 



The Laminariales comprise the kelps, the largest 

 of the brown algae. They are widely distributed 

 throughout temperate and arctic regions, occurring 

 mainly in cool waters and making their greatest dis- 

 play along shores bordering the North Pacific Ocean. 

 Most of the Laminariales grow below the low-tide 

 line. They include about 30 genera and 100 species. 

 Some of the best-known members are Laminaria, 

 Macrocijstis, Nereocystis, Postelsia, and Egregia. 

 Laminaria, with 30 species, is the largest genus. 



Laminaria. Common along both coasts of North 

 America, in cooler waters, are various species of 

 Laminaria. Some are not more than 30 cm. long, 

 while others reach a length of 9 to 12 m. They live 

 attached to rocks just below the low-tide line. Alter- 

 nation of generations is heteromorphic. The large 

 vegetative plant is a sporophyte (Fig. 63). It consists of a long blade 

 and a thick leathery stipe anchored by means of a branching basal 

 holdfast. According to the species, the blade may be entire or divided 

 lengthwise into segments. The cells of the stipe show a differentia- 



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Fig. 63. Laminaria, 

 a small kelp with a 

 blade, stipe, and 

 holdfast, about one- 

 half natural size. 



