326 Pub. Paget Sotind Biol. Sta. Vol. 2, No. 53 



A. Frond much flattened, blade-like; midrib evident (ligiilate species). 

 B. Frond unbranched. 



C. Stipe short (4-5 mm.), heavy; secondary veins not pro- 



nounced • • D. pinnatinervia Mont. 



CC. Stipe long (10-20 mm.), heavy; secondary veins pro- 

 nounced, appressed D. tabacoides Okam. 



CCC. Stipe long (8-16 mm.), slender; secondary veins pro- 

 nounced, at about 90 degrees D. foliacea n. sp. 



BB. Frond branched. 



D. Branches of first and occasionally second order; plant 



body large, with main axis up to 1 m. wide, 8 m. long- 



D. latissima Setch. and Gard. in litt. n. sp. 



DD. Branches of third and fourth orders; plant bod}^ small. 



E. Fronds u}) to 8 nun. wide. Z). ligulata (Lightf.) Lamour. 



EE. Fronds 10-30 mm. wide. D. herbacea (Turn.) Lamour. 



AA. Frond terete or compressed, not flattened ; midrib not evident. 



(Filiform species). 



3. INIORPHOLOGY 



a. Historical 



There are two types of thallus in the genus, represented by Des- 

 marestia aculeata and D. ligulata. In the D. aculeata, or filiform type, the 

 main axis and all its branches, to the last order, are slender and terete, 

 or only slightly compressed, with the axial filament and its surrounding 

 tissues, the so-called midrib, not at all evident from the surface. In the 

 D. ligulata, or ligulate type, the main axis and all its branches are much 

 flattened, with a very evident midrib. The latter type, the ligulate group, 

 represented on the Pacific coast according to the opinion of the writer by 

 four species, will be considered in the following discussion. 



The general growth habit of all species studied is the same, and has 

 already been worked out by various writers. The plan of the present 

 writer has been to gather together from many scattered sources and to 

 summarize the work already done and then to add the results of her own 

 observations. 



The plant is attached to its substratum, which may be a rock, a shell, 

 or the holdfast or stipe of another alga, by means of a simple disk-shaped 

 oi flattened-conical holdfast. From the center of the holdfast arises a 

 single round or slightly compressed axis which begins to widen out within 

 1 or 2 cm. into the ligulate thallus. The main axis may remain un- 

 branched, as in D. foliacea, or may bear branches to the fourth order, as in 

 l). ligtdata. Lateral branches are opposite, distichous, patent to erect, with 



