XVII.] ANNUALS AND PSEUD -ANNUALS. 295 



"When perennial plants are taken northwards, there is 

 a tendency for them to produce bulbs or seeds rela- 

 tively earlier in their life history than they do in their 

 normal environments ; so, whilst the i)lant may be 

 killed by frost, it has already provided means for re- 

 producing it^ kind when congenial conditions again 

 appear. The longer such plants are grown in the 

 northern station, the more completely do they adapt 

 themselves to the short season, not only by confirming 

 the habit of early production of bulbs or seeds, but 

 by an increasing tendency to early maturation of the 

 plant as a whole. Yet the cases are comparatively 

 rare in which a perennial plant becomes a true annual 

 at the north, — that is, one in which the whole plant 

 matures before frost. The tomato is often cited as 

 an example, for it is perennial or at least plur- annual 

 in its original home ; but it generally grows in the 

 north until killed by frost, and does not ripen up 

 completely with the normal progression of the sea- 

 son. The egg-plant, red pepper, cotton and castor - 

 bean are similar instances of plants which become 

 potential annuals, or plur -annuals, at the north by 

 adapting themselves to the shorter seasons by means 

 of hastening their life epochs (see page 45). 



It is a nice question as to just what elements of 

 climate are responsible for this adaptive change, and 

 what their exact impressions are upon the physio- 

 logical processes of the plant ; but this adaptation 

 to environment in cultivated plants is undoubtedly 

 guided by the same laws which determine the gen- 

 eral distribution of plants in the various life -zones. 

 The general limits of floral and faunal zones are 

 probably closely associated with relative temperature, 



