216 



INTRODUCTION TO CETPTOGAMIC BOTANY. 



in this converge ; thus affording a ready method of ascertain- 

 ing the genus in default of fructification." Harv. Mel. p. 100. 





^rnr 



Fig. 55. 



a. Tip of young frond of Bictyota dichotoma. 

 h. Cell showing spiral structure. 



c. Threads produced from marginal cells in young fronds. 



d. Sorus of spores. 



e. Terminal cell in act of division. 



f. Ditto with a view to make two new centres of growth. 



All magnified.* 



Zonaria affords the well-known Turkey feather or Peacock's 

 tail layer of our southern coasts and of North America. The 

 species are generally remarkable for their wedgeshaped fronds?, 



* I do not find the division of the cells as regular as figured by 

 Nageli, (Die neuern Algensysteme, tab. 5, fig. 10 — 21,) who makes each 

 circle of cells increase in geometric progression, nor do they all grow 

 in regular lines. Great difierenees, in this respect, will be found in 

 difierent specimens, or in different parts of the same specimen. Nageli 

 was, however, the first to show the mode of growth. When the frond 

 becomes forked, the termuial cell divides longitudinally, and then each 

 half cell grows according to its own law. As the base of the new ceU 

 assumes a circular outline, the cells which rise from its concentric and 

 radiate division assume also a circular disposition, being as it were 

 meridians of latitude to the meridians of longitude. The frond of 

 Bictyota is peculiarly liable to be infested with species of Cocconema, 

 which under a low magnifier look like little plant-lice. The calcareous 

 Algse are also liable to the same affection. 



