48 SEAWEEDS 



since varieties of ordinarily vesicular species are 

 known to occur without such floats. In some genera 

 (Halidrys, Sargassum, and its allies) they occur in 

 definite positions, but in other genera capriciously, 

 and may be derived from parts of the stem as well 

 as from the leaves. Rosanoff states that they con- 

 tain nitrogen, while more recent research (Wille, loc. 

 cit.) adds oxygen, and denies the presence of carbonic 

 acid. 



The reproductive organs of the Fucacece occur 

 on specialised branches in the most highly dif- 

 ferentiated forms, such as Turbinaria, Sargassum, 

 &c., and again in one of the least developed in a 

 vegetative sense, viz. Himantlialia, which from an 

 insignificant button-shaped thallus sends forth fertile 

 branches several feet in length. They are sometimes 

 to be found (e.g. Coccophwa) on the air-vesicles only, 

 or on them as well as elsewhere. Frequently they are 

 confined to the thickened ends of leaves or branches 

 (e.g. Fucus, &c.) and more rarely (Durvillea, Sarco- 

 pliycus) scattered indiscriminately over large portions 

 of the thallus, or as in Myriodesma over the whole 

 thallus, except the root-disc. All degrees are re- 

 presented, from the highest, imparting to the shoot 

 the appearance of an inflorescence, to the simplest, in 

 which no part is differentiated for the bearing of 

 reproductive organs. 



The reproductive organs are borne in conceptacles, 

 which are more or less globular or ellipsoidal cavities 

 situated beneath the surface of the tissue, and com- 

 municating with the outside through a narrow open- 

 ing, the ostiole. These cavities originate near a 



