EUPHYCOPHYTA 233 



narrow assimilatory hairs interspersed with large colourless cells. 

 These two types of epidermal cell are apparently differentiated 

 near the apex of the thallus, the small ones giving rise to hairs, 

 monosporangia or antheridia. The large colourless cell is said to 

 form a protection against intense light, but it may also be a relic 

 of a tissue which formerly had a function that has since been lost. 

 One or two spores are formed in each monosporangium, whilst 

 the spermatia arise in sori, forming bunches of cells at the ends of 

 the small-celled assimilatory branches. The carpogonial branch is 

 three-celled, the reproductive cell containing two nuclei, one in the 

 carpogonium proper and one in the trichogyne. The second cell of 

 the carpogonial branch gives rise to a group of four large cells 

 which are rich in protoplasm, whilst the sterile envelope of the 

 cystocarp arises from the third cell. 



After fertilization the zygote nucleus passes into one of the four 

 large cells where meiosis then takes place. Three of the nuclei 

 degenerate and the remaining one passes into the single initial 

 gonimoblast filament which later becomes branched. There is 

 therefore no diploid phase since meiosis occurs immediately after 

 fertilization. 



Chaetangiaceae : Galaxaura (galax, milky; aura, gold). Fig. 129 



This is a genus that is widely distributed in the tropics. The 

 thallus is more highly differentiated than the preceding example 

 because the outermost cells of the lateral branches unite to form a 

 cortex. The thallus is more or less heavily calcified and may be 

 jointed due to periodic loss of meristematic activity in the apical 

 cells. New meristematic tissue forms beneath and bursts through 

 the old tissue. The principal interest of the genus hes in its repro- 

 duction. For a long time it had been known that there were 

 apparently only sexual or asexual species, differing morphologi- 

 cally in habit and in the asexual plants having one more cell layer 

 in the cortex. Owing to the work of Svedelius it is now known that 

 in this genus a delay in reduction division has occurred so that the 

 carpospores are diploid and give rise to tetrasporic plants. The 

 whole carpogonial branch acquires diploid nuclei and so forms a 

 complex of auxihary cells. There is thus an alternation of unlike 

 generations, though the difference is largely in appearance rather 

 than in size. This behaviour is so anomalous as compared with 



