OF PERIODIC PHENOMENA. 353 



year round ; or which yield varieties by cultiva- 

 tion, varying much in their time of flowering, so 

 as to render it necessary to observe always the same 

 variety, which is often impossible ; or which have 

 other species so nearly allied to them, as to make 

 it difficult for observers in general to distinguish 

 between them ; or which, lastly, are of such a kind 

 that the exact period of the flowers opening does 

 not admit of being determined with precision ; such 

 are the Illecebrum, Aquilegia, and others. 



(15.) With due regard to the above points, the 

 selected species are for the most part perennial or 

 woody ; these last deserving especial notice from 

 their serving also for observations in respect of leaf- 

 ing. The list comprises plants from all the European 

 families, preference being given to those which are 

 most common, or which are most widely distributed, 

 and which have large-sized flowers ; they are also so 

 chosen as to offer species, some of which flower every 

 month in the year. There are some precautions to 

 be observed with respect to the particular individuals 

 on which our observations are made. Thus, we 

 should avoid those plants which have been planted 

 within a year previous ; as transplanting in the 

 spring leads to some uncertainty in the time of leaf- 

 ing and flowering the season following. We should 

 also, for reasons already given (9.), make our observa- 

 tions, as much as possible, on the same individuals 

 every year; which should be selected according to 

 the circumstances under which they grow. On 

 these accounts it is judged preferable to observe cul- 

 tivated plants, in most instances, rather than wild 



