44 SEAWEEDS 



petiole, as exhibited by Turbinaria, Sargassum, &c., 

 the appearance of the whole plant (conf. Fig. 7a, b.) 

 by no means suggests the familiar definition of a 

 thallus. (The strict justification of Prof. Bower's 

 proposal to employ the terms phyUidium 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 Fucacece 

 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 Fucacece by the 

 active division of the outermost parenchyma cells, 

 and may go so far as to represent considerable 



