lyS THE ALGAE 



the degree of differentiation is remarkable. So far as the lamina is 

 concerned the group is usually regarded as primitive because the 

 new portions do not originate separately but by intercalary growth 

 from an existing portion (cf. Fig. 97). 



The sporangia and paraphyses are borne in irregular or more or 

 less regular sori on both sides of the lamina. They arise from super- 

 ficial cells that grow out from the surface. These divide tangentially 

 into a basal cell and paraphysis initial, and later from the basal cell 

 a sporangium is cut off. In L. saccharina the sporophytes only 

 reach maturity at the end of their second year, and for sori to de- 

 velop the distal tissue of the frond must be in at least the sixth 

 month of growth. The actual number of reproducing plants and 

 the duration of the reproductive period in a population depend 

 largely on the numbers of the different season and age groups 

 making up the population. In general L. saccharina in Great 

 Britain fruits in summer, L. clous toniin winter, and L. digitata from 

 April to November with maxima in spring and autumn. 



The unilocular sporangia usually each give rise to thirty-two 

 zoospores, meiosis taking place in the process. It is probable that 

 the zoospores possess an eye spot, but it must be very small be- 

 cause in the three species where it has been recorded it was very 

 difficult to distinguish. The zoospores, which in one or two cases 

 are reported to be of two sizes, germinate to form minute gameto- 

 phjrtes, but on germination they first put out a tube that terminates 

 in a bulbous enlargement into which the contents of the zoospore 

 migrate. There the nucleus divides and one daughter nucleus passes 

 into the tube whilst the other degenerates, but at present the sig- 

 ficance of this phenomenon is obscure : it would hardly seem to be 

 associated with meiosis because this process takes place in the 

 zoosporangium. Both kinds of gametophyte show much variation 

 in shape and size, the male gametophyte being the smaller through- 

 out as it is built of smaller cells that contain dense chromatophores. 



The gametophytes can be cultivated in the laboratory, but for 

 successful cultivation the water must be sterilized and the cultures 

 placed close to a north window in winter and 2 or 3 m. distant in 

 summer. Reproductive organs are only formed at low temperatures, 

 2-6° C, whilst above 12-16° they are rarely produced, this fact 

 perhaps accounting for their temperate and arctic distribution (cf. 

 Fig. 100). It is also known that the eggs may develop parthenogene- 

 tically to give a haploid sporophyte which has an irregular shape, 



