178 PHAEOPHYCEAE 



Subsequent growth of the stipe takes place in the outer layer 

 of the medulla, and in the adult organ five regions can be recog- 

 nized : 



(i) Primary fixing organ. 



(2) The bulb. 



(3) A flattened twisted area said to provide additional rigidity. 



(4) A portion with flounced edges. 



(5) A flat straight portion that passes into the lamina. 



The existence of these structures is supposed to be correlated 

 with the large lamina which is cleft into many linear segments. 



If, as sometimes happens, the whole of the plant is torn away with 

 the exception of the bulb, this organ is still capable of reproduction 

 and assimilation. The advanced external diflFerentiation of the stipe 

 is not reflected in its histology where the diflFerentiation is poor 

 because there is no secondary growing region, no mucilage ducts, 

 and trumpet hyphae are not conspicuous. 



Saccorhiza and Alaria are the only two genera in the Lami- 

 nariales with cryptostomata that are at all comparable to those of 

 the Fucales (cf. p. 194), the former genus possessing true crypto- 

 stomata with tufts of hairs. There are three theories concerning 

 the homologies of the cryptostomata which may be mentioned 

 briefly here (cf. also p. 196): 



(i) They are incomplete sexual fucoid conceptacles which have 

 failed to develop. 



(2) They are forerunners of the sexual fucoid conceptacle. 



(3) They are a parallel development with the sexual conceptacles 

 of the fucoids, but otherwise have no relation to them. 



Whilst there is very little evidence for any one of these theories it 

 may be suggested that the second alternative probably fulfils most 

 nearly the known facts. 



The male gametophyte is filamentous whilst the female fre- 

 quently consists of only one cell which functions as the oogonium. 

 After fertilization has taken place the development of the sporo- 

 phyte to maturity in both species requires only one year so that the 

 plants are true annuals. Saccorhiza bulbosa is found on the Atlantic 

 coasts of north and west Europe whereas the other species, S. der- 

 matodea, is circumpolar and is possibly the parent species from 



