FUCACE/E. 115 



but in Himanthalia lorea the vegetative portion of the 

 thallus commences as small pear-shaped bodies, attached by 

 a small disc. Eventually the upper portion becomes cup- 

 shaped, and during the second year a long, narrow, dichotom- 

 ously-branched receptacle springs from the depressed centre 

 of the basal, cup-like portion. The branches of the recep- 

 tacle are of nearly uniform width, varying from one-sixth to 

 one-quarter of an inch, and sometimes reaching a length 

 of fifteen feet or more. The " gulf-weed " of the " Sargasso 

 Sea" is not attached, and floats in dense masses on the 

 surface of the sea, being buoyed up by numerous air- 

 bladders. 



The tissue of the thallus shows a considerable amount of 

 differentiation. The central portion consists of a so-called 

 medullary system, composed of loosely compacted, elongated 

 cells, and surrounded by a cortical layer of short cells, 

 which is again surrounded by an epidermis of small cells that 

 are closely compacted. Increase in length depends on the 

 repeated division of a single pyramidal or four-sided apical 

 cell ; increase in thickness depends on the radial division of 

 the outermost cortical cells. In some species the flattened or 

 leaf-like portions of the thallus are of equal thickness 

 throughout; in others, as the species of Sargassum and 

 Fucus, there is a central thicker portion or "midrib." 



Protoplasmic continuity is very clearly shown in most 

 species. 



Air-bladders or floats are present in most of the species. 

 These may occur in the substance of the frond, as in Fucus 

 vesiculosus and Ascophyllum nodosum. In the gulf-weed, 

 Sargassum bacciferum, they are globose and stalked, while in 

 Halidrys siliquosa they are long, pod-like, and constricted 

 at intervals. 



