FUCALES (HETEROGENERATAE) 203 



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. Growth curves show that the greatest length is 

 attained by these annual fronds on plants growing in the lowest 

 part of the dense zone and that the shortest occur in the highest. 

 This can be correlated with (a) the greater degree of desiccation at 

 the higher levels, and (b) the fact that the less frequent flooding 

 reduces the supply of available salts. Reduction has proceeded so 

 far in this genus that only one ovum matures in the ripe macro- 

 sporangium. The liberation of the gametes is controlled by the tides 

 and exposure and there is a definite periodicity related to these t^vo 

 factors. 



Cystoseiro-Sargassae 



Sargassaceae : Halidrys (halt, sea; drys, oak). Fig. 136. 



The perennial fronds arise from a conical holdfast and bear 

 pedicelled air vesicler, but as these are lanceolate and jointed they 

 probably represent a series of vesicles. There are only two species, 

 the European i7. siliquosa being hermaphrodite whilst the Californian 

 one is dioecious. In both, the stalked receptacles form terminal 

 racemes at the apices of branches, but only one ovum matures in 

 each macrosporangium. In the Californian H. dioica there are a 

 number of interesting morphological features : 



{a) An unbroken series can often be found which shows every 

 gradation between a leafy member and the series of vesicles. 



(h) Protoplasmic connexions between cells are continuous 

 throughout the whole of the plant, a feature which should be 

 compared with the condition commonly found in the Rhodo- 

 phyceae (cf. p. 212). 



[c] The origin of the vesicles appears to be largely dependent 



upon the food supply. 



{d) The cells in the centre of the mid-rib have definite sieve 

 plates comparable to those in the trumpet hyphae of Laminaria, 

 though without the bulbous swellings. 



(e) The air chambers and primary hyphae appear to arise in 

 regions which are losing their vitality, though the significance of 

 this behaviour is not clear. 



