192 



ALG.K 



in the Nemalieo2, the apical cell appears to be replaced by a group of equi- 

 valent cells, ^^1lle distinguishes six types of Floride?e as respects their 

 mode of growth ; in four of these groups growth takes place from a 

 single apical cell ; in two from an apical mass of cells, with peripheral 



growth. In the so-called 

 polysiphonous seaweeds, 

 such as Polysiphonia 

 (Grev.), a transverse sec- 

 tion of the 'frond' shows 

 a large central elongated 

 cell, surrounded by four 

 or more smaller cells, 

 which are also elon- 

 gated in the longitudi- 

 nal direction, and which 

 are known as siphons. 

 These perice?itral tubes 

 are often connected with 

 one another and with 

 the axial cell by threads 

 of protoplasm. When 

 the pericentral tubes are 

 wanting the frond is 

 monosiphonous. In some 

 genera belonging to the 

 Ceramiace^ either the 

 single axial tube or both 

 axial and pericentral 

 tubes are surrounded by 

 a pseudo-cortex formed 

 of a dense agglo- 

 meration of secondary 

 branches originating at 

 the nodes and closely 

 adpressed to the pri- 

 mary branch. These 

 are always the result of 

 further division of the 

 pericentral tubes, the 

 apical cell remaining 

 undivided. In other genera a similar cortical tissue is composed of 

 moniliform rows of cells arranged at right angles to the axis. 



Although there is in the Florideae no distinct differentiation of the 



Fig. 169. — Gigartina maviillosa Ag., with cystocarps 

 (natural size). (After Luerssen.) 



