CHLOROPHYCE 147 
which unite in the centre above the insertion of 
the stalk. This central area immediately above 
the stalk is covered with a flat circular membrane 
above, and is surrounded, where the radial chambers 
are given off, by a continuous circular cushion (Fig. 
43c), the corona superior. This corona consists of as 
many segments as there are radial chambers, and 
each segment bears the scars of hairs that have 
fallen off or remain incompletely developed. In like 
fashion below the marginal radial chambers there is 
another cushion, the corona inferior (Fig. 43c), corre- 
sponding with the upper one, but bearing no hair- 
scars. A lower but less-marked cushion exists, but 
it gradually becomes merged in the central area. 
Each marginal ray stands in communication with the 
relative segments of the corona (superior and in- 
ferior), and these in turn are marked off from the 
central area by a fold of the membrane with a 
central opening, which frequently becomes closed 
by subsequent thickening. The radiating chambers 
bear the spores (gametangia). 
In the species formerly placed under Polyphysa 
(which has been merged into Acetabularia by Graf 
zu Solms-Laubach), the sporangial rays are com- 
pletely free from each other, and not united into a 
firm disc, as in A. mediterranea, They are quite un- 
calcified in some species and very slightly so in 
others and have no corona inferior, while the corona 
superior is represented by free knobs bearing hairs. 
A. mediterranea takes several years to attain the 
formation of a fertile cap. In the first season no cap 
is produced, but merely erect stalks, with occasion- 
L 2 
