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 Phaeophyceae. 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. 



