272 



Siphonocladiales 



being shed, rings of scars are left on the main axis. After several years of 

 sterile growth the plant eventually develops a fertile disc. In the radiating 

 branch-segments of the fertile disc numerous oblong-ellipsoid coenocysts (often 

 called ' aplanospores ') are formed (fig. 175 G apl). These are ultimately set 

 free after which they rest for three months, and then become converted into 

 gametangia, each of which opens by a lid at one end, liberating numerous, 

 small, biciliated isogametes. These conjugate in pairs (fig. 175 DF) to 

 form zygotes which after a resting period germinate directly into new plants. 



Fig. 175. A, Halicoryne Wrightii Harv., upper portion of thallus, x58; /, whorl of fertile 

 branches; g, branch-segment in which the cyst-like 'aplanospores' are formed; st, whorl of 

 sterile branches. B G, Acetabularia mediterranea Lamx. J3, upper part of thallus in 

 median section to show branches, x 5 ; st, sterile whorl ; sc, superior corona ; ic, inferior 

 corona; g, fertile branch-segment. C, upper part of another individual showing formation 

 of 'aplanospores' (apl), x 3. D F, conjugation of gametes, x 450. G, ' aplanospore ' 

 ( = gametangium) x 100. (A, after Cramer, from Wille; J5, after Nageli from Oltmanns; 

 C G, after Wille.) 



In Acetabularia crenulata two or three fertile whorls of branch-segments 

 are formed successively above each other. These form basin-shaped discs, 

 and a fertile disc of a similar shape occurs in A. Caliculus. In the latter 

 species the ' aplanospores ' (gametangia) are spherical and there is no resting 

 period, the gametes being set free while the gametangia are still within 

 the radial branch-segments (Borgesen, '13). 



The genus Acicularia differs from Acetabularia in the calcification of the 

 walls of the ' aplanospores/ which adhere together as a calcareous mass. This 

 genus, which was first described from fossil forms, has one living representative, 

 Acicularia Schenckii (Mob.) Solms. Its interesting history has been well 

 summarized by Seward ('98). In Halicoryne there are alternate whorls of 

 sterile and fertile branch-segments (fig. 175 A), the former polytomously 



