174 THE ALGAE 



sporophyte depend on season of zygote germination, depth in the 

 sea and type of habitat. Thus winter sporophytes rarely attain 

 maturity and the bulk of the populations in Great Britain originate 

 in the spring, except on sheltered coasts where summer plants 

 thrive equally well. The total life span of this species rarely exceeds 

 three years but that of some others, e.g. L. cloustoni, may be longer. 

 Growth is seasonal, more rapid growth occurring between January 

 and June and slower growth afterwards. Latitude may exert an 

 important influence here, because in the Barents Sea the blade of 

 L. digitata only grows from spring to July whereas the stipe grows 

 all the summer. The rhizoids or crampons, however, attain their 

 maximum growth in the autumn. In L. saccharina and L. cloustoni 

 the seasonal changes in growth rate are indicated in the stipe by 

 alternate zones or ^rings' of lighter and darker tissue, the darker 

 being formed during the periods of slow growth. In L. saccharina 

 the maximum height and greatest growth is achieved during the 

 second rapid growing season. 



Many of the species are used as food by the Russians, Chinese 

 and Japanese. The greatest number of species occur around Japan 

 and from Alaska to CaHfornia. Furthermore, these kelps, as they 

 are called, are valuable as a source of iodine and as a potassic 

 fertiHzer (see pp. 445, 449). 



The following brief notes concern a few species that are of more 

 general interest: 



L. cloustoni: The attachment crampons are arranged in four 

 lateral rows and there is a long rough cyUndrical stipe with an 

 abrupt transition to the lamina. When bent, the frond and stipe 

 tend to snap. 



L. saccharina: The margin is thicker than the central part of the 

 thallus and the wavy lamina is produced by continual growth of the 

 central portion without any growth in the marginal areas. The 

 stipe is short and the transition to frond is gradual. 



L. digitata: In this species the transition from stipe to frond is 

 gradual. In contrast with L. cloustoni the stipe is smooth and 

 neither it nor the frond snap when bent over. 



L. ephemera: A species closely allied to the above but differing 

 from it in that there are no crampons but only a basal attachment 

 disc. 



Morphologically both lamina and stipe are divided into four 

 regions. On the outside there is the actively dividing meristoderm 



