290 BOARD OF AGRICULTURE. 



growth every summer. The succulent prickly pear survives the 

 alternate freezing and thawing to which it is exposed on the 

 sands of Nantucket, while the guiacum, which produces the solid 

 lignum vita;, succumbs to tlie slightest frost. In like manner 

 the tuberous roots of the dahlia are killed by frost while those 

 of the peony are uninjured. The cormus and foliage of the 

 delicately beautiful crocus seem utterly indifferent to the effects 

 of cold, wliile those of the elegant ixia, belonging to the same 

 family, are exceedingly sensitive — a difference of habit wliich the 

 keenest botanist would fail to discover, except by actual trial. 

 The relations of living plants to drought and moisture are 

 equally remarkable and inexplicable. Tims most seeds and 

 many bulbs will retain their vitality unimpaired for years in a 

 dry atmosphere, whjle others, like the seeds of the magnolias 

 and the nymplieas and the scaly bulbs of the lilies, germinate 

 with great difficulty after desiccation. 



In the study of the vegetable kingdom we find some species 

 of a truly cosmopolitan character, and existing in a wild state 

 in all quarters of the globe. The common brake, pteris aqui- 

 lina, is a striking instance of this sort, being abundant in vari- 

 ous parts of the five grand divisions of the earth and on many 

 islands of the ocean. Tliis is doubtless owing to its vigor of 

 constitution and the minuteness of its spores, which are borne 

 to great distances by the winds. About thirty species of flow- 

 ering plants are common to both the arctic and antarctic regions 

 and are also found upon numerous intervening mountain sum- 

 mits. "Whether they are the remnants of an ancient flora which 

 once covered the whole earth, and which has been largely super- 

 seded by species of more recent origin, or whether they were 

 created in many widely separated localities simultaneously, are 

 questions which we have not the data to answer. 



The area over which any particular wild species is distributed ' 

 is usually definitely limited, and the centre of greatest abun- 

 dance from which it seems to have spread is well marked. 

 Under similar climatic influences in different countries, we do 

 not generally discover the same identical species, but in most 

 cases such as are closely allied. Thus the violets of Europe 

 are not repeated in America ; but we have an abundance of 

 similar species. The heaths of Europe are represented by dif- 

 ferent species in Africa, and by a (Jiffereut but strikingly similar 



