EUPHYCOPHYTA 99 



gametes which develop in gametangia that are cut off from the 

 parent thallus by means of septa. Both types of gamete are bi- 

 flagellate, but the microgametes differ from the macrogametes in 

 that they lack pyrenoids. The gametes are liberated through gela- 

 tinization of the apex of the gametangium, and after fusion has 

 taken place the zygote germinates at once into a new plant. The 

 plants are diploid because meiosis takes place at gamete formation; 



Fig- 55 Bryopsis. a, plant of 5. plumosa ( x o-6). b, portion of same 

 ( X 7). c, B. corticulans, rhizoid formation from lower branches. 

 d, the same, B. scarf ei. e, B. plumosa, female gemetangia. f-h, stages 

 in septum formation at base of gametangium. g= gelatinized 

 material, m = membrane, r = ring of thickening initiating septum, 

 (a, b, after Taylor; c, d, after Chapman; e-g, after Fritsch.) 



there is therefore no haploid generation. The plants can behave like 

 Vaucheria (cf. p. 265) in their response to certain environmental 

 conditions ; thus, gamete formation is hastened by transference of 

 the plants from light to dark or by changing the concentration of 

 the nutrient solution. Inversion of the thallus takes place under 

 conditions of dull hght or when it is planted upside down, and 

 under these circumstances the apices of the pinnae develop rhizoids. 

 This exhibition of polarity indicates clearly that the thallus is dif- 

 ferentiated internally, but it is still a matter for speculation as to 

 how such differentiation can occur in an organism which is to all 

 intents and purposes one unit. 



