92 



THE ALGAE 



ently not changed very greatly: a large number of fossil forms are 

 known (cf. pp. 297-30). The genera are primarily confined to 

 warm waters and in many there is extensive calcification. 



Dasycladaceae: Dasydadus (dasy, hairy; cladus, branch). Fig. 51 



The family Dasycladaceae is very ancient and was formerly much 



more widely spread since sixty fossil genera are known whilst there 



m 



P p:^ m 





Fig. 51 Dasydadus clavaeformis. A, plants. B, assimilatory fila- 

 ments showing mode of branching. C, gametangium {g). D, 

 thickenings at base of assimilatory filaments, h = point of origin of 

 branch, /=base of lateral, // = calcified wall, m thickened base of 



wall. (After Fritsch.) 



are only ten living today (cf. p. 297). Dasydadus forms dense 

 growths, up to 5 cm. in height, in shallow waters where the plants 

 are anchored by means of richly branched non-septate rhizoids. 

 The central axis bears dense whorls of profusely branched laterals 

 which are arranged alternately above each other. The branches 

 arise in whorls of four immediately below the apex of the parent 

 cell, to which they are united by narrow constrictions, and although 

 the rest of the main axis is impregnated with Ume throughout there 

 is none at the constrictions. If the axis of 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. 



