Fig. 1. — Geranium syhaticion, 



CHAPTER I 



INTRODTJCTIOK 



OxjR knowledge of the origin and past histoiy of our 

 British Flora is still very incomplete. The researches 

 of geologists, however, and especially of Mr. Clement 

 Reid, have made it probable that before the glacial 

 period the flora of the British Isles did not materially 

 differ from the present. Some, at any rate, of our 

 northern forms were already here, as well as other more 

 southern species, such as the Hawthorn (Crataegus) 

 and some, such as the Spruce Fir, which were driven 

 away, or rather killed out, by the cold, and have never 

 returned. Some of our plants have been introduced by 

 man intentionally, and others, the so-called weeds of cul- 

 tivation, unintentionally. Some which might be thought 

 to be thoroughly established — the Lime, for instance, — 

 though they produce abundance of fruit, never or scarcely 

 ever reproduce themselves by self-sown seed. Even the 

 Common Elm is said to produce perfect seed two or 

 three times only in a century. 



The northern limit of plants is not determined merely 

 by the average temperature ; it depends on the respective 



iE B 



