292 



THE ALGAE 



nected by single fine protoplasmic threads, though actual proof of 

 linkage has not so far been obtained. Reproduction is by means of 

 hormogones, which are only produced in the uniseriate lateral 

 branches. Heterocysts are also present. 



* Stigonemataceae : Stigonema (stigon, dotted; Tiema, thread). 

 Fig. 173 



The threads consist of a single row or multiple rows of cells with 

 definite apical growth, enclosed within a gelatinous sheath which 

 varies in thickness and colour. Shortly behind the apical cell one of 

 the products divides twice to give a cruciform group. One of these 

 cells gradually moves into the centre and by further division a 

 central cell surrounded by four pericentrals is produced. Most peri- 

 centrals divide radially so that ultimately successive tiers of 7-8 



Fig. 173 Stigonema minutum. h =heterocyst ( x 170). (After 



Fremy.) 



