206 THE ALGAE 



during which period it is carried up the edge of the groove to the 

 side of the thallus by the activity of the primary apical cell. The 

 limiting layer immediately around Ai does not undergo further 

 growth and so it also comes to He in a groove. Later on, tertiary 

 (AAi) and quaternary (AAg) apical cells are cut off from Aj, the 

 tertiary cell becoming the apical cell of a sterile or fertile branch. 



The free-living salt marsh forms of Ascophyllum nodosum ap- 

 parently arise as a result of alternating high and low salinities such 

 as one finds in estuaries. Ecad scorpioides requires long periods of 

 low saUnity and ecad mackaii short, frequent periods. The same 

 interpretation may apply to var. minor but, since it is turf-living, 

 there is only rain water to lower the salinity. In ecad mackaii cessa- 

 tion of bladder formation is brought on by continuous submergence 

 (though it does not survive if the sahnity is below 2 per cent), lack 

 of Hght, development of apical receptacles and reduced salinity. 



Himanthaliaceae: Himanthalia (himant, thong; halia^ of the sea). 

 Fig. 116 

 The short, perennial frond or button arises from a small disc-Hke 

 holdfast, the shape of the button being dependent upon level 

 because it is short and stumpy when it grows exposed at high levels, 

 whilst it is more elongate at the lower levels where the plants are 

 submerged for longer periods. From March to July of each year 

 new receptacles grow out from the centre of the buttons and form 

 very long strap-shaped and repeatedly forked structures filled with 

 mucus. Although the button is regarded as the frond and the thong 

 as the receptacle, it is possible to regard the button as part of the 

 stipe and the thong as frond plus receptacle, as in Durvillea. In 

 support of the second interpretation it should be mentioned that 

 the receptacle contains horizontally running hyphae which have so 

 far only been found in Durvillea in receptacular regions. In the 

 receptacles these horizontal hyphae appear to be associated with 

 the formation of air spaces. Growth curves show that the greatest 

 length is attained by these annual thongs on plants growing in the 

 lowest part of a dense zone, and that the shortest occur in the 

 highest. This can be correlated with (a) the greater degree of desic- 

 cation at the higher levels, and (b) the less frequent flooding re- 

 ducing the supply of nutrient salts. The apical groove, containing 

 the apical cell and hairs, is perpendicular to the plane of flattening 

 and not parallel. Internally it has been found (Naylor, 1951) that 



