1 84 



THE ALGAE 



branched structures from each of which hang 100-150 laminae that 

 bear sporangia in longitudinal folds when they are mature. Apart 

 from the cryptostomata of Saccorhiza and Alaria it has also 

 been suggested that the occur- 

 rence of these sporangia in folds 

 may illustrate how the fertile 

 fucalean conceptacle may have 

 arisen. Such a change would 

 necessitate the development of 

 wedges of sterile tissue in order 

 to divide up the folds, but 

 whether such a change could 

 occur in a relatively differen- 

 tiated thallus is a matter for 

 speculation. 



The numerous laminae are 

 formed by a spUtting process in 

 which a portion of the lamina 

 fails to continue growth whilst the 

 rest goes on growing, and in this 

 manner a weak area is formed 

 from which a split commences. 



Lessoniaceae : Nereocystis (nereo. 



Nereis, daughter of Nereus; 



cystis, bladder). Fig. 105 

 The plants, which from the re- 

 corded observations appear to be 

 annuals, may attain a maximum 

 length of 90 m., but 50 m. is more 

 usual. This bears a bladder up to 2-3 m. in length^ which has a 

 capacity of several Htres. The long, slender stipe is soUd and cylin- 

 drical below but swollen and hollow above, finally contracting just 

 below the terminal spherical bladder which bears a row of short, 

 dichotomous branches, each giving rise to a number of long, thin 

 laminae. The plant commences with only one blade which divides 

 twice in a dichotomous fashion, thus producing four blades, and 

 these form the centre of activity for the remainder through a pro- 

 cess of sHtting. The spUtting of these four fronds is preceded by 



^ Generally much smaller. 



Fig. 104 Postelsia palmaeformis. 

 (After Oltmamis.) 



