81 



slits. Its air-floats are pear-shaped, and the power of 

 flotation is so great that large stones to which it 

 grows attached are sometimes wrenched free and 

 thus transported. The greatest girth of stem is 

 attained by Lessonia. 



The holdfasts are in most cases strong rootlets, 

 but in Saccorhiza (" sea-furbelows ") whorls of tentacu- 

 la or haptera are developed after the primary hold- 

 fast. These originate immediately above it from a 

 swelling, the rhizogen, which in S. bulbosa becomes 

 enlarged into the bulb characteristic of this form. 



Several distinct tissues, as might be expected, go 

 to compose the thallus of the Laminariacece and in 

 this respect their differentiation recalls that of the 

 Fucacece, while in some respects it surpasses it in 

 degree. The epidermal layer consists of cells slightly 

 elongate in shape and containing chromatophores. 

 It is the assimilative tissue in the leaves and in the 

 stalks as well, though in the latter case it often loses 

 this character and assumes that of a meristematic 

 layer bringing about the secondary growth in thickness 

 of the stalk. However, in other forms this secondary 

 growth in thickness is the function of a special 

 peripheral meristem, which, as in the higher plants, 

 adds centripetally to the thickness of the internal 

 tissues of the stalk and at the same time centrifugally 

 to an external bark-like tissue. Within the epidermal 

 layer there is a parenchymatous layer of thin-walled 

 cells, which composes the greater part of the leaf- 

 tissue and a considerable portion of that of the 

 stalk. Bordering this tissue internally there occurs 

 in the stalks another layer of elongate cells with 



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