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, Joc. 
cit.) adds oxygen, and denies the presence of carbonic 
acid. 
The reproductive organs of the Fucacee 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. Himanthalia, which from an 
insignificant button-shaped thallus sends forth fertile 
branches several feet in length. They are sometimes 
to be found (e.g. Coccophora) 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- 
phycus) scattered indiscriminately over large portions 
of the thallus, or asin 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 cavitics originate near a 
