Apical Growth in the Genus Trentepohlia. 243 
through the previous layer which covered the apex, so that the cell- 
wall at the extreme apex has but one thin layer of cellulose and a 
curious apical cap. This process is continually going on as the 
apical cell grows in length, and the structure of the cell-wall can be 
seen by reference to Fig. 2. 
Fig. 2. Semidiagrammatic figure of the structure of the wall of the 
apical cell of T. Montis-Tabula var. ceylauica. The innermost layer of cellulose 
(l) is filled in black, tip. cp., apical cap. 
The apical cap fits firmly over the end of the cell, covering the 
delicate end-wall. It is a secretion of the apical cell, and can be 
seen in all stages of growth, the first indication of its formation 
being quite evident at the apex of even the youngest branches. 
The newly formed cap is homogeneous, consisting of one layer of 
substance, but older caps may consist of two or more layers, which 
in many cases become partially free. Thus, the free end of a cell is 
sometimes surmounted by a successive series of apical caps (Figs. 3 
and 4). Occasionally the caps appear to be raised on a short 
pedestal of the same substance. 
When removed, the cap is seen to be hat-shaped or bonnet¬ 
shaped, thick in the flattened middle portion and rapidly thinning 
down to a fine, deeply serrated fringe at the downwardly turned 
edges (Fig. 4 C). This fringe surrounds the apex of the cell and fits 
very closely over the terminal portion of the thick side-walls, thus 
keeping the cap in position. 
