6 Brannon.' — The Siriichtre and Development of 



General Histology. 



The entire surface of the plant is covered with a thick 

 gelatinous secretion, which causes it to adhere firmly to paper 

 when mounted. 



The vegetative cells, forming a single layer in the foliar 

 expansion of the frond, are polygonal in shape, have thick 

 cell-walls, protoplasm peripherally arranged containing gra- 

 nular material, a large nucleus, and a great number of red 

 chromatophores (Fig. i/). These chromatophores are also 

 found in the superficial cells of the midrib, the stalk and the 

 upper surface of the holdfast. The cells in the apical and 

 marginal (merismatic) portions of the frond are smaller, have 

 more delicate cell-walls, and are far more active than those in 

 the middle and basal regions of the frond. 



The midrib is three to five layers of cells in thickness. The 

 central cells are very large, elongated, and angular. The super- 

 ficial cells are much smaller and more circular than those in 

 the centre. All of them have a thick cell-wall and a thin, 

 peripheral layer of protoplasm containing a large amount of 

 granular material (Fig. i m). 



Transverse sections of the dense disk-shaped holdfast show 

 that it is composed of a central region of large oval and oblong 

 ceils surrounded by layers of cells which gradually diminish 

 in size and increase in numbers toward the surface of the 

 organ, where they are quite uniformly roundish and form 

 a strong cortical region (Fig. %). Surrounding the entire 

 holdfast is a very thick yellowish coat of cellulose, through 

 which project many small rhizoidal filaments. These fila- 

 ments are composed of one to many rows of thin-walled cells, 

 and serve to absorb food-material as well as to fix the plant 

 more firmly to its substratum. The cells of the holdfasts are 

 completely filled with protoplasm and granular material ; have 

 a gelatinous, cellulose cell-wall, and are connected with one 

 another by protoplasmic pits. 



Sections through the holdfast, midrib, and frond show 

 protoplasmic pits connecting adjoining joint-cells. It is not 



