40 8 Osterhout . — On the Life- Hi story of 
axis of the frond. At a short distance from the growing- 
point it is impossible to distinguish the central filament 
from the cells which surround it. The cells occupying the 
interior of the frond finally become long, narrow tubes, much 
curved and bent, resembling those of the medulla of Fucus. 
The highly refractive plates between the cells become 
developed to an unusual degree (Fig. 10 at the left). In 
this manner a medulla arises composed of a varying number 
of tubular longitudinal filaments, which anastomose laterally 
in an irregular manner, and lie imbedded in a gelatinous 
substance which fills the central portion of the frond. The 
medulla traverses the frond throughout its entire length. 
The cells lying externally to the medulla constitute the 
cortex. Those forming the exterior of the frond are relatively 
small and elongated in the radial direction (Fig. io). They 
are densely filled with protoplasm, and have large, deeply- 
coloured chromatophores. They form the most actively 
assimilating layer. Passing inward towards the medulla, 
the cells of the cortex grow rapidly larger, becoming 
rounded or ellipsoid. At the same time they grow poorer 
in protoplasmic contents, and the chromatophores become 
paler and more reduced the nearer the cells lie to the 
medulla. 
Physiologically considered, the relations between the cells 
are not unlike those of a radial leaf, especially one with 
a poorly-developed central-bundle system. The actively 
assimilating outermost layer, composed of cells directed 
perpendicularly to the surface, corresponds to the palisade- 
layer. The rounded cortical cells have large intercellular 
spaces filled with water containing the necessary gases for 
the metabolism of the innermost cells of the interior, much 
as the spongy parenchyma provides for the circulation of 
gases in the leaf. The inner cortical cells also serve to 
convey elaborated products to the elongated cells of the 
medulla, which probably convey them to the growing-point 
and also to ' the holdfast, where they are stored up as 
Floridean starch. Wille ( 85) mentions Rhabdonia as having 
