200 THE ALGAE 



inner cells of the cortex, the latter tissue being responsible for the 

 formation of the secondary hyphae (cf. Fig. 113) which penetrate 

 between the primary medullary hyphae and finally outnumber 

 them. There is a greater development of secondary thickening in 

 the stipe and mid-rib than there is in the frond, whilst in very old 

 parts of the thallus the limiting layer may die off and then the under- 

 lying cortical cells take over its function. Development of secondary 

 hyphae is most pronounced near the basal disc, which ultimately 

 consists of a mass of hyphae. 



Growth in length takes place by means of an apical cell which 

 lies at the bottom of a slit-like depression that has resulted from the 

 more rapid growth of the surrounding limiting layer. The apical 

 cell is three-sided in young plants whilst in the adult thallus it be- 

 comes four-sided, the new segments being cut off successively from 

 the base and four sides, after which they develop into the various 

 tissues (Fig. iii). Injury, and also the stimulus provided when 

 the thallus Hes on marsh soil, induces new growth in the neigh- 

 bouring cells, and in this manner proliferations are formed which 

 may also serve for vegetative propagation. Both cryptostomata 

 and conceptacles arise as depressions in the surface of the 

 thallus, and it is now known that they can arise in one of two ways 

 (Fig. 112): 



(i) A linear series of two or more cells is formed, but their hori- 

 zontal activity then ceases, thus leaving a terminal initial cell 

 which becomes sunk in a depression as the surroimding 

 tissues grow up. In this type the sides of the conceptacle are 

 derived from the limiting layer and underlying cortex, e.g. 

 Fucus, though in Himanthalia only the Umiting layer is in- 

 volved. Pelvetia canaliculata forms an exception in that the 

 first division is vertical and not horizontal. The floor of the 

 conceptacle, however, is derived by division of the original 

 basal cell of the hnear series. Finally, around the remnants of 

 the one or more initial cells, a central mucilaginous column 

 is formed, stretching to the neck of the conceptacle and con- 

 nected to the walls by thin strings of mucilage, which are 

 later ruptured. 



(2) The conceptacle develops from a single initial that divides 

 transversely into two unequal cells, the upper or tongue cell 

 degenerating, whilst the lower one gives rise to the walls of 

 the conceptacle by lateral divisions except at the very top 



