ECOLOGICAL FACTORS 



355 



properties of the cell wall, although it is also possible that these 

 features have appeared as a result of the habitat they occupy. 



Of those members of the Fucaceae which appear in belts, 

 Pelvetia canaliculata, which forms the highest zone, is subject to the 

 greatest exposure, but the situation of the algae in relation to each 

 other and the density of the flora will also affect the water loss. 

 Actual measurements carried out in the field show that the loss of 

 weight curves for this alga are characteristically hollow, the 

 greatest loss being in the first 6 hours, whilst the total loss may be 



3 4 5 6 



^ Time in Hours 



Fig. 204. Loss of water, as represented by loss in weight, in Pelvetia canaliculata 

 during intertidal exposure. (After Isaac.) 



between 60 and 68 % during periods of 8-9 hours (cf. fig. 204). 

 Fucus spiralis f. platycarpus shows the same order of water loss as 

 Pelvetia but then it only occupies a slightly lower belt. Besides 

 being able to tolerate a considerable water loss which may enable it 

 to live in a relatively inhospitable habitat, the plants of Pelvetia^ in 

 order to succeed, must be able to reproduce under such conditions. 

 The two ova are not liberated from the thick-walled mucilaginous 

 megasporangium as they are in Fucus and so the antherozoids must 

 penetrate this envelope, and although this structure secures the 

 protection of the eggs under desiccating conditions there is 

 apparently no protection for the antherozoids. We do seem to be 



23-2 



