THE ALGAE 139 



The young plant has a group of apical hairs, formed at the growing point, 

 each of which has a basal region of active cell division. At a later stage one 



Cortex Ostiole Conceptacle Medulla 



Fig. 127. — Fucus vesiculosus. Transverse section through 

 fertile tip of frond showing cortex, medulla and 

 / numerous conceptacles. 



or more of these hairs, usually that in the centre, dies off to its base, and the 

 basal cell gives rise to the apical cell of the future thallus. 



When the thallus is injured the inner tissue near the wound grows out 

 into a tuft of hairs and the same process is repeated, producing an apical cell 

 from which new tissues are regenerated. 



Scattered along the length of the thallus in close association with the 

 thickened midrib are air bladders,* which serve very much the same function 

 as the air spaces which we shall meet with in certain aquatic flowering plants, 

 namely, that of assisting the plant to float. 



Small tufts of hairs grow from cells in the apical groove and also from 

 small cavities, called cryptostomata, scattered over the surface of the thallus. 

 These hairs may act as absorbing organs. 



Structure of the Thallus 



The anatomy of the frond of Fucus shows little difference in structure 

 wherever we may cut it, that is to say, that except for a variation in the 

 quantity of the tissues the structure is the same throughout the length of the 

 thallus. The external layer consists of small rectangular cells with abundant 

 plastids, which give them a dull brownish colour, and it is in this layer that 

 the bulk of the assimilation takes place. The cells of this layer remain capable 

 of active division all over the plant. Within this external layer comes a varying 

 thickness of cortex composed of thin-walled parenchymatous cells. These 

 also continue to divide actively, so that the cortex increases in thickness 

 towards the base of the plant. The inner cortical cells also grow out into 

 thick-walled hyphae, which grow in among the original filaments of the 

 central zone or medulla, filling this space with a mass of closely interwoven 



* These are absent in some species. 



