INTRODUCTION. 11 



their growth, and are hence considered rare plants. Thus, for 

 example, there are certain species which require that the air sur- 

 rounding them should contain a minute quantity of salt, dissolved 

 in its moisture ; these only abound, therefore, near the sea- 

 shore ; but they are seen to spring up in the neighbourhood of 

 salt-works, or on lias soils which contain a good deal of saline 

 matter, even many hundred miles inland, their seeds being con- 

 veyed by the wind or by birds, which have spread them over 

 the whole surface of the earth, but there only meeting with the 

 conditions they require for their development. On the other 

 hand, there are many which can grow in almost any situation, 

 and which can adapt themselves to a great variety of circum- 

 stances, often exhibiting evident changes of form and aspect, 

 which are due to the influence of these. Such are common 

 plants ; and many of them are among those most serviceable to 

 man, on account of the improvement which can be effected in 

 them by cultivation. , For example, the Potato, growing in its 

 native climate the tropical portion of South America does 

 not require for the growth of its young shoots that store of 

 nourishment which, in temperate climates, is provided in its 

 fleshy tubers ; and the edible portion is thus extremely small, 

 since the warmth and moisture constantly supplied to it de- 

 velop the growing parts without such assistance. But when 

 transplanted to colder regions, and to a richer soil, that store is 

 greatly increased in amount, and becomes one of the most im- 

 portant of all articles of food to man. If it were not for this 

 capability of adapting itself to new circumstances, the plant 

 could not thrive in Northern Europe ; ' since its own powers of 

 growth would be insufficient, when the external conditions are 

 so much changed. But it is this very capability which renders 

 it so useful to man. If the large Potatoes of European culti- 

 vation be planted again in tropical climates, the produce is little 

 superior to that of the original stock ; since, when circumstances 

 no longer demand it, the acquired habit ceases. The Cabbage, 



