SECT. II. 



PEACOCKS-TAIL LAYER. 



247 



division of which at the lower side, the other cells 

 of the frond are formed, the terminal cell of the frond 

 being thus continually pushed onwards. Hence it re- 

 sults that the longitudinal lines of superficial cells con- 

 verge, thus affording a ready method of ascertaining 

 the genus in default of fructification. When a new- 

 centre of growth is to be made, that is, when the frond 

 is to become forked, the terminal cell divides longitu- 

 dinally and then each half-cell grows according to its 

 own law. Fig. 30 shows the tip of the frond of the Dic- 

 tyota dichotoma magnified; the cells on its surface 

 are square, and the 

 interior of each has 

 a spiral structure. 



The Padina Pa- 

 vonia, or Peacock's- 

 tail laver of our 

 southern coast, and 

 those of North Ame- 

 rica and the Mediter- 

 ranean, is sometimes 

 included in the ge- 

 nus Zonaria. The spe- 

 cies is remarkable 

 for its wedge-shaped 

 fronds, which are 

 olive green shaded with rust colour, and, when in 

 fruit, they are striped across with dark concentric zones, 

 which are merely lines of spores immersed in the frond 

 and seen through its transparent superficial membrane. 

 Each zone is ornamented with a fringe of orange-coloured 

 hairs. Parallel to, or rather concentric with, the spores, 

 is a row of articulated threads, which bear so strong 

 a resemblance to the antheridia of the Cutleria that 

 a similarity of function is suspected by Mr. Berkeley. 

 Species of Zonaria, Padina, and Haliseris, which is the 



Fig. 



Dictyota dichotoma : a, tip of young frond ; 

 6, cell showing spiral structure; c, threads from 

 marginal cells ; d, sorus of spores ; e, /, terminal 

 cells dividing into new centres of growth. 



