LIFE HISTORIES OF ALG^E 231 
1. The cortex, composed of several external layers of 
cells, somewhat resembling, in arrangement, the palisade 
parenchyma of the leaf and, like the latter, having the 
function of photosynthesis. The outer portion of this 
layer is further loosely differentiated into an epidermoidal 
tissue, but there is no true epidermis. The outer cell- 
walls of this layer, forming the external surface of the 
plant, possess a thick layer of cuticle. The cells of the 
cortex retain their embryonic character for a long time, 
and by successive divisions favor the growth of the branch 
in thickness. 
2. The medulla, or central tissue, is composed of cells 
arranged for the most part in rows, so as to form filaments. 
This tissue serves chiefly for the conduction of liquids. 
The walls of its cells are mucilaginous and much swollen, 
except at certain small pits, the "sieve tubes," closed by 
a perforated membrane. 
210. Photosynthesis. The cells of the cortex possess 
several chromoplasts or chromatophores, each containing 
chlorophyll, by which photosynthesis is possible; but, in 
addition to chlorophyll, the chromatophores contain also 
a brown pigment (phycophain) , which masks the chloro- 
phyll, and explains the external color that gives the name 
Phaeophyceae to the family. 
211. Vegetative Multiplication. Vegetative multiplica- 
tion does not occur in Ascophyllum, nor in most of the 
genera of Phseophyceae. In the few genera where it has 
been observed, it is accomplished by a fragmentation of the 
plant body, or by the death of the older part of the thallus. 
The pieces thus set free may develop new plants, but these 
usually remain sterile. 
