NEMALIONALES 221 



and possess a trichogyne which shrivels away after fertihzation. 

 The nucleus of the fertilized carpogonium divides twice, thus 

 giving rise to four cells, and from these the short gonimoblast 

 filaments grow out and finally terminate in a sporangium that 

 produces a single naked carpospore which soon secretes a cell. 

 A character of many of the Nemalionales is the occurrence of 

 meiosis immediately after fertilization so that the carposporophyte 

 is haploid as in the Proto-florideae. 



The life history of the related genus Nemalion is similar to that of 

 Batrachospermum, except that when the spermatia are liberated 

 the nuclei are often in prophase, the division being completed when 

 they have become attached to a trichogyne. This division has 

 suggested to some workers that the spermatium is really homo- 

 logous to an antheridium, but it might also be argued that it is a 

 relic of a time when an antheridium produced more than one 

 spermatium. 



*Chaetangiaceae : Scinaia (after D. Scina). Fig. 145. 



This is a widespread genus with its home primarily in the 



northern hemisphere, the commonest species, S. furcellata, being 



monoecious, although one may find monospores and spermatia on 



the same plant. The fronds, which arise from a discoid holdfast, 



are subgelatinous, cylindrical or compressed and dichotomously 



branched. The centre of the thallus is composed of both coarse and 



fine colourless filaments, the former arising from the apical cell and 



the latter from the corticating threads. There is also a peripheral 



zone of horizontal filaments that terminate in short corymbs of 



assimilatory hairs with a large colourless cell in the centre. These 



two types of epidermal cell are apparently differentiated near the 



apex of the thallus, the small ones giving rise to hairs, monospor- 



angia 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 auxiliary cells 



