' i*-. ' f. T- r 



''iff'" 



.i-^-yi^-:^ 



'■■>i,- ' ■ ■ .'■'.' ■ 



•• • . '^-^VV- : ■ .• • ; -—■;• v:".:i>. V ■• vo: M .•i-'.-i?.-Viv;; V-iv;.-'.: " • ■^V.f..-: :iv ■.■/I'-.-.^Cfff .... * 

 ..-■ i.i'\i'-{-.. l-;>-' v',.-*^ '. 



B 



Fig. 151 Trihonema. A, T. homhycina ( x 450). B, T. minus, hypno- 

 spores. C, D. construction of H piece in T. hombycina as shown 

 after treatment with KOH ( x 675). (A, C, D, after Smith; B, after 



Fritsch.) 



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

 Desmidiaceae. Each cell contains one nucleus, although Trihonema 

 homhycina may have two together with two or more parietal chloro- 

 plasts. Asexual reproduction is by means of zoospores (two to four 

 per cell) which are Hberated by separation of the two halves of the 

 cell. On coming to rest the zoospore elongates and puts out an at- 

 tachment process, and in this state it much resembles Characiopsis. 

 Aplanospores (one to two per cell) and akinetes, which are formed 

 in chains, also act as additional means of propagation, whilst sexual 

 reproduction is apparentiy very rare, when some of the motile 

 bodies come to rest first and are surrounded by other motile 

 gametes. Iron bacteria sometimes live symbiotically with this alga 

 and colour it yellow or brown from ferric carbonate. This sub- 

 stance controls the pH of the water and thus acts as a local buffer 

 for the alga whilst the bacteria obtain their oxygen requirements 

 from the Trihonema. 



Phyllosiphonaceae : Phyllosiphon {phyllo^ leaf; siphon^ tube). Fig. 

 152 

 This is an endophytic alga that occurs in the leaves and petioles 

 of the Araceae, most of the species being confined to the tropics, 

 although one is found in Europe, including Great Britain. Recent 

 work has shown that the food reserve is oil and not starch, and that 

 the pigments are not those associated with the Chlorophyceae. For 

 these reasons the genus has been removed from its former place in 



263 



