44 : SEAWEEDS 
petiole, as exhibited by Turbinaria, Sargassum, &e., 
the appearance of the whole plant (conf. Fig. 7a, 0.) 
by no means suggests the familiar definition of a 
thallus. (The strict justification of Prof. Bower’s 
proposal to employ the terms phyllidium and 
caulidium when speaking of the oophyte generation 
in contrast to phyllome and caulome of the sporophyte, 
must be admitted, but the advantage of such a 
usage in morphological argument does not exclude 
the propriety of employing the more familiar terms 
when it is expedient.) Greater diversity of appearance 
is imparted by the transformation of leaves into air- 
vesicles, formed by a rending apart of the central 
tissue in young leaves. In this process the fragments 
of broken-down cells are left behind in the growing 
cavity, and, at a later stage, the cells bordering the 
interior form by division a secondary dermal layer. 
In the mature vesicles a cuticle separates from the in- 
ternal surface, exposing a layer of large papillate cells. 
Fig.7,g. The tissues of the higher forms of the Fucacee: 
are composed of three distinct layers in stem, petiole, 
and leaf, viz., an epidermal layer of generally narrow, 
radially elongated cells, and beneath it a band of thick- 
walled cortical parenchyma passing into a central 
strand. The epidermal layer is assimilative in its 
function, and varies in depth from one to several rows 
of cells. The outermost cells are capable of division, 
and by this means, dividing radially and tangentially, 
effect an increase in the thickness of the shoot. A 
kind of periderm is formed in some Fucacew: by the 
active division of the outermost parenchyma cells, 
and may go so far as to represent considerable 
