PHHOPHYCEA 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. dudbosa becomes 
enlarged into the bulb characteristic of this form. 
Several distinct tissues, as might be expected, go 
to compose the thallus of the Zaminariacew and in 
this respect their differentiation recalls that of the 
Fucacee, 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 
G 
