SIPHONOCLADIALES 



83 



segments which are morphologically equivalent to the primary- 

 branches. The basal segments also bear on their upper surface 

 small projections, with or without hairs, which form the corona, 

 whilst in one section of the genus there is also an inferior corona on 



Fig. 57. Acetabularia. A, plant of A. crenulata ( x o-8). B, apex of A. medi- 

 terranea showing corona. C, apex of A. Moebii showing two superposed fertile 

 whorls. D, A. mediterranea, attachment rhizoid and perennating vesicle {b). 

 E, A. crenulata, cells near centre of thallus, showing superior corona (c) and leaf 

 scars (5). F, A. pusilla, vegetative ray segment ( x 44). G, fertile lobes of A. 

 Schefikii with cysts ( x 44). H, cysts in A. pusilla in a single lobe of the um- 

 brella ( x 37). I, single cyst of A. mediterranea. J, young plant in first year. 

 K, L, A. crenulata, apices of ray segments ( x 37). M, A. crenulata, superior 

 corona ( x 37). N, A. crenulata, inferior corona ( x 37). (A, F-H, K-N, after 

 Taylor; B-E, I, J, after Fritsch.) 



the lower surface. In A. mediterranea two or three years elapse 

 before the plant attains to maturity. In the first year the branched 

 holdfast produces an upright umbilical thread, together with a 

 thin-walled lobed outgrowth that penetrates the substrate in order 

 to function as the perennating organ. The aerial part dies, and in 



6-2 



