354 



ECOLOGICAL FACTORS, ETC. 



the higher the alga grows the greater is the fat content (p. 288), and 

 hence the thickness of the cell wall must be of some significance. 



84 96 Hours 



Fig. 203 . Loss in weight of Fucoids in relation to time of desiccation. The higher 

 an alga grows the slower it loses water and the greater the total loss. Symbols the 

 same as in Fig. 202. (After Zanefeld.) 



Subsequent examination has shown that the thickness of the cell 

 wall does bear a relation to the height at which an alga grows. 



Table XXI 



Fucus spiralis 

 Ascophyllum nodosum 

 Fucus vesiculosus 

 Fucus serratus 



Thickness in 

 divisions of 3 /x 



0-49 ±0-05 

 o-34±o-oi 

 0-23 ±0-03 

 0-I4 + 0-0I 



These cell walls decrease in thickness when subjected to desiccat- 

 ing conditions, and the higher a fucoid is growing on the shore the 

 more the cell walls shrink on drying ; so it must be assumed that a 

 large part of the water lost is contained in the cell walls (cf. fig. 

 202 B, C). Those species which lose water most slowly will also 

 reabsorb it most slowly and, as a result, the growth rate of the 

 highest species will therefore tend to be the slowest. It would 

 appear from this study that the real factor controlling zonation, so 

 far as the fucoids are concerned, is the biochemical nature and 



