CHLOROPHYCE^E 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 coi^ona 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 PolypJiysa 

 (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 



