MARINE ALG^E OF BEAUFORT, N. C. 461 



in conspicuous roundish or ellipsoidal son, scattered over both surfaces ; oogonial son 

 black, antheridial sori whitish. 



About 37 species in warm and temperate seas, one extending to Scandinavia. 



Dictyota dichotoma (Hudson) Lamouroux. PI. XCIV, figs, i, 2 c and d, and 3. 



Uha dichotoma, Hudson, 1762, p. 476. 

 Dictyota dichotoma, Lamouroux, 1809, p. 42. 

 Dictyota dichotoma, Harvey, 1852, p. 109. 

 Dictyota dichotoma, De Toni, 1895, p. 263. 

 P. B-A. Nos. 282, 1641,2175. Fasc. E. No. CXX. 



Frond erect, flat, ribbonlike, sometimes narrowed at the base to a very short stipelike portion, 

 attached by a small, padlike thickening; regularly dichotomous, sometimes with irregular branches 

 given off from the apices and from the margins; margins smooth, entire; apices usually rounded, obtuse, 

 sometimes rather acute; tetrasporangia, andoogonial and antheridial sori scattered all over both surfaces 

 except base, tips, and margins; tetraspores produced continuously, not in regular crops; oogonia and 

 antheridia produced in crops at regular intervals; sexual and asexual plants showing a regular alternation 

 of generations. 



Reported from warm and temperate waters generally, extending in Europe as far north as Norway 

 and Helgoland. 



Very abundant on Fort Macon and Shackleford jetties, Beaufort, N. C., and in harbor from low 

 water to i m. below low water, and occasionally abundant on Bogue Beach, June to October; fairly 

 abundant in Newport River near "Green Rock"; abundant in North River off Lennoxville and Mar- 

 shallburg; one small mass floating in Core Sound off Davis Island; two plants 2 cm. long on coral reef 

 off Beaufort, N. C., May, 1907, and fairly abundant, August, 1914 and 1915. Abundant in sound near 

 Moores Inlet, Wrightsville Beach, N. C., July to September, 1909. 



This is the northern known limit in North America of the species and of the genus. 



The species varies considerably in size, width, amount of branching, and acuteness of apices, vary- 

 ing from plants i to 3 mm. wide and 6.5 cm. long to plants 4 to 16 mm. wide and 29 cm. long. The 

 average of six well-developed plants from Beaufort was 4 to 12 mm. wide, 18 cm. long. The.branching 

 may be frequent, forming a short, dense habit, or may be infrequent, forming a long, open habit. The 

 apices, while usually obtuse, may be acute. The Beaufort plants, while varying in these respects, 

 show less variation than English specimens. 



All the specimens of this species dredged from the coral reef, August, 1914, were very narrow and 

 finely divided, with numerous almost linear proliferations (Plate XCIV, fig. 2 c and d). 



Plants from unfavorable situations are narrow, often spirally twisted, and usually small. The 

 apices of these plants are often acute. Some apices of larger plants may be acute at times, since, when 

 conditions are changed to less favorable ones or sometimes after fruiting, there are formed narrow pro- 

 jections from the apices. These may widen out later or may grow out as proliferations from the apices, 

 later widening out and branching dichotomously. Plants collected at the beginning of this process, 

 if examined by themselves, would often be determined as D. bartayresiana Lamour. Under different 

 conditions of growth plants may resemble D. bartayresiana Lamour., D. divaricaia Lamour., D. dichotoma 

 f. latifolia (Kuetz.) Vinassa, f. attenuata (Kuetz.) Vinassa, or f. implexa (Lamour.) Vinassa. These 

 three last-named forms are at Beaufort only growth forms occurring under different conditions in the 

 environment. D. bartayresiana can itself not be sharply distinguished from D. dichotoma, since 

 specimens of these species may overlap. Many specimens of D. dichotoma from England are narrower 

 and more acute at the apices than shown in photographs of the type of D. bartayresiana. 



D. dichotoma, wherever carefully observed, has been found to produce its sexual cells in regular 

 periodic crops. In the three European stations where this process has been studied Bangor, Wales, and 

 Plymouth, England (Williams, 1905), and Naples, Italy (Lewis, 1910) the plants produce two 

 crops a month at regular intervals related to the tidal seasons, the relations of the crops to the tides 

 varying in the different localities. At Beaufort (Hoyt, 1907), only one crop a month is produced, this 

 being initiated from three days before up to the day of the greatest springtide at the time of the full 

 moon, as shown by the tide tables, and being liberated from three to six days after the day of the greatest 

 springtide. The relation between the greatest springtide and the times of initiation and liberation of the 



