6 Brannon^ — The Structure 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 f). 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. 3 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. 2 ). 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 
