SIPHONOCLADIALES 



8i 



throughout there is none at the constrictions. If the axis or a 

 branch is decapitated a new apex is regenerated, whilst if a rhizoid 

 is cut off and inverted it develops a normal apical cell. Short- 

 stalked spherical gametangia arise at the apices of the major 

 branches in the upper half of the plant and are cut off by a septum. 

 The plants are essentially dioecious and produce isogametes that 

 sometimes exhibit relative sexuality. 







Fig. 55. Dasycladus clavaeformis. A, plants. B, assimilatory filaments showing 

 mode of branching. C, gametangium (g). D, thickenings at base of assimilatory 

 filaments. 6 = point of origin of branch, / = base of lateral, // = calcified wall, 

 m = thickened base of wall. (After Fritsch.) 



Dasycladaceae : Neomeris {neo, new; meris, part). Fig. 56. 



This is a calcareous tropical genus which has been in existence 

 from the Cretaceous era. The much calcified adult plants have the 

 appearance of small worm-like masses with an apical tuft of hairs, 

 whilst very young plants consist of an erect Vauchena-\\k.Q. filament 

 with a tuft of dichotomously branched filaments at the apex. In the 

 adult plant the ultimate branches terminate in long deciduous hairs, 

 whilst the apices of the next lower order of branches dilate and 

 become pressed together, thus producing a compact surface with 

 a pseudo-parenchymatous appearance (cf. fig. 56 E). Calcium 

 carbonate is deposited wherever there is a mucilage layer and an 

 aggregation of the chloroplasts, but apparently both these con- 

 ditions must be fulfilled before lime can be laid down. The 

 principal interest of this form lies in its morphological resemblance 

 to certain fossil genera (cf. p. 271). 



CSA 6 



