1925] Setchcll-Gardner: Melanophyceac 651 



Key to the Geneka 



1. Growth in length by division of an apical cell 59. Dictyota (p. 651) 



1. Growth in length by the division of many marginal cells 2 



2. Frond with a distinct midrib 60. Neurocarpus (p. 655) 



2. Frond without a midrib 3 



3. Reproductive organs on both sides of the frond 5 



3. Reproductive organs on only one side of the frond 4 



4. The terminal margin inrolled 64. Padina (p. 661) 



4. The terminal margin not inrolled 62. Chlanidophora (p. 658) 



5. Sori usually in more or less distinct concentric lines partially embedded in the 



frond, even at maturity .61. Taonia (p. 656) 



5. Sori seldom concentric, entirely superficial at maturity 63. Zonaria (p. 659) 



59. Dictyota L amour. 



Frond plane, membranaceous, ecostate, dichotomous below, some- 

 what irregularly cleft and flabellate above, arising from a stupose base, 

 and consisting of two layers of cells, an inner layer of large, colorless, 

 longitudinally elongated, prismatic cells, and a cortex of small, assimi- 

 lating cells, arranged longitudinally in rows; reproduction sexual, by 

 antheridia developed in small groups producing antherozoids with a 

 single terminal cilium, and by oogonia in groups, each oogonium pro- 

 ducing a single egg which is extruded and fertilized in the water ; and 

 asexual by aplanospores, 4 in a sporangium, cruciately divided, 

 developed from the surface cells, scattered, sparse. 



Lamouroux, Nouv. Bull. Soc. Philom., vol. 1, 1809 (May), p. 331, 

 and in Desv., Jour, de Bot., vol. 2, 1809a, p. 38. 



The older genus Dictyota was distinct among the Dictyotaceae 

 whose fronds, arising from a single apical cell, were flattened or com- 

 pressed and more or less dichotomously branched. The latest divisions 

 of J. G. Agardh, while attractive and representing certain tendencies 

 toward increase in complexity, yet present certain difficulties in the 

 line of cleavage. Dilophus, for example, presents species in typical 

 form with two or four layers of cells throughout the older portions of 

 the frond, but what is to be done with those species which are two- 

 layered only on the extreme margins, those which are two-layered 

 more widely on the margins, and those which are only one-layered 

 occasionally ? All of our species are of the first or second types and 

 we feel better satisfied to refer them all to the genus Dictyota. Con- 

 cerning the other segregate of J. G. Agardh (1894) from the Dictyota 

 of earlier accounts, we are in no position to speak. 



