XANTHOPHYCEAE 



117 



are said to give rise to motile gametes, although this is a feature that 

 requires further investigation. 



*Tribonemaceae : Tribonema (tribo, thin; nema, thread). Fig. 82. 



This is a filamentous analogue to a form such as Microspora 

 (cf. p. 46) with which it is frequently confused. T. bombycina 

 sometimes appears in sheets covering ponds and pools and if these 

 dry up they form an algal ''paper". The unbranched threads are 

 composed of cells possessing walls of two equal overlapping halves, 

 with the result that the filaments are open-ended and tend to 

 dissociate into H pieces. At cell division a new H piece arises in the 



■■■■n^'I^' 



D 



B 



Fig. 82. Tribonema. A, T. bombycina (X450). B, T. minus, hypnospores. 

 C, D, construction of H piece in T. bombycina as shown after treatment with 

 KOH (X675). (A, C, D, after Smith; B, after Fritsch.) 



centre and the two halves of the parent cell separate, somewhat as 

 in the Desmidiaceae. Each cell contains one nucleus, although 

 Tribonema bombycina may have two together with two or more 

 parietal chloroplasts. Asexual reproduction is by means of zoo- 

 spores (two to four per cell) which are liberated by separation of the 

 two halves of the cell. On coming to rest the zoospore elongates 

 and puts out an attachment process, and in this state it much re- 

 sembles Characiopsis. Aplanospores (one to two per cell) and 

 akinetes, which are formed in chains, also act as additional means of 

 propagation, whilst sexual reproduction has been seen only once 

 when some of the motile bodies came to rest first and were sur- 

 rounded by other motile gametes. Iron bacteria sometimes live 



