EUPHYCOPHYTA 253 



Rhodomelaceae : Lawr£m;/a (after H. de Lalaurencie). Fig. 143 



These plants, which are cartilaginous, initially have the same 

 structure as Polysiphonia^ but this can only be observed at the apex. 

 The apical cell produces three segments all of which form richly 

 branched trichoblasts that are arranged spirally. Beneath each tri- 

 choblast two pericentral cells are cut off and by subsequent divisions 

 of these the compact cortex is produced, so that not far behind the 

 apex the basic structure has become lost. The apical growing cell is 

 situated in an apical pit from which a tuft of hairs projects. The 

 reproductive organs are much as in Polysiphonia except that the 

 wall of the cystocarp consists of more than two layers. The fertile 

 tips of male plants also swell up because when it becomes fertile 

 the apical cell ceases to divide whilst surrounding cells continue to 

 do so. The genus is widespread in cold and warm waters, the species 

 often occurring in rock pools or near low water mark. 



Delesseriaceae : Delesseria (after Baron Delessert). Fig. 144 



The large, thin, leafy fronds, which are bright red in colour, 

 possess a very conspicuous mid-rib with both macro- and micro- 

 scopic veins. The complex nature of the laciniate or branched 

 thallus can be seen from the figure. The foUar condition is clearly 

 produced by the juxtaposition in one plane of numerous branches 

 that arise from the axial filament. The apical cell divides to give a 

 central cell and two pericentrals. The latter divide and give rise to 

 the pinnate laterals of the first order on which secondary laterals 

 are borne. The mid-rib is formed from the central cell and the ad- 

 jacent cells of the primary laterals. The latter divide transversely 

 so that in the mature frond they are half the length of the central 

 cells. Growth is intercalary in the various orders of branches. The 

 cells of the thallus also become united by means of secondary proto- 

 plasmic threads and they may also develop thin rhizoids. The 

 reproductive organs are borne on separate adventitious leaflets. 

 The procarps arise from the axial threads of the leaflet, each cell 

 giving rise to two procarps. The procarp consists of a support cell 

 which then cuts off a sterile cell and a three-celled carpogonial 

 branch. The second cell of the fertile branch is much enlarged. As 

 many as fifty procarps may be produced on a fertile leaflet. These 

 ripen in acropetal succession but only one is fertilized. At fertiliza- 

 tion a second sterile cell is cut off" and the other adjacent procarps 

 and sterile cells give rise to the wall of the cystocarp. 



