32 PLANT-ANIMALS [CH. 



six hours' light-exposure followed by a long spell of 

 eighteen hours' dark-exposure. But and the fact is 

 remarkable these conditions of light and darkness 

 are precisely those to which C. roscoffensis is exposed 

 during the spring tidal periods at which its eggs are 

 laid habitually. At such periods, low water of suc- 

 cessive tides occurs about the middle of the day and 

 of the night, and hence, in twenty-four hours, the 

 C. roscoffensis zone is uncovered once during day- 

 time and once during night-time. So it comes about 

 that, during the spring tides, C. roscoffensis is exposed 

 for about six hours to the light and for the rest of 

 the twenty-four hours it is in darkness. Therefore, 

 as jthe laboratory experiments show, of all the daily 

 changing light conditions to which it is subjected 

 throughout a lunar period, those which obtain at 

 spring tide are most favourable to the deposition of 

 egg-capsules. 



In ascribing to light a leading rdle in determining 

 the periodicity of egg-laying we have the support of 

 not a few well-established biological facts. Thus the 

 profound influence which light exerts on plants, both 

 on their development in general and on their flower- 

 production in particular has long been recognised. 

 Perhaps the best-known example of this influence 

 is afforded by the common ivy. It is a fact of 

 general observation that ivy growing on a wall 

 rarely if ever flowers, though when climbing over 



