LENGTH OF THE DAY. 37 



evening, and closing in the morning ; restored to 

 the open air, she recovered her usual habits. 



Tropical plants in general, as is remarked by 

 our gardeners, suffer from the length of our sum- 

 mer daylight ; and it has been found necessary 

 to shade them during a certain part of the day. 



It is clear from these facts, that there is a 

 diurnal period belonging to the constitution of 

 vegetables ; though the succession of functions 

 depends in part on external stimulants, as light 

 and heat, their periodical character is a result of 

 the structure of the plant ; and this structure is 

 such, that the length of the period, under the 

 common influences to which plants are exposed, 

 coincides with the astronomical day. The power 

 of accommodation which vegetables possess in 

 this respect, is far from being such as either to 

 leave the existence of this periodical constitution 

 doubtful, or to entitle us to suppose that the day 

 might be considerably lengthened or shortened 

 without injury to the vegetable kingdom. 



Here then we have an adaptation between the 

 structure of plants, and the periodical order of 

 light and darkness which arises from the earth's 

 rotation ; and we find, moreover, that the arbi- 

 trary quantity in the two laws, the length of the 

 cycle of the physiological and of the astronomi- 

 cal fact, is the same. Can this have occurred any 

 otherwise than by an intentional adjustment? 



Any supposition that the astronomical cycle 



