THE CURRENTS OF THE ATLANTIC. 399 



life of the plant is impossible ; the latter are the limits which de- 

 termine its flowering, and the ripening of its seed or fruit. 



It is hardly needful to remind you, that the very existence of 

 plants is dependent upon two extremes of temperature, one higher 

 and the other lower. In the temperate regions of the earth, in 

 which we ourselves have the happiness to live, the effect of the 

 higher extreme is seldom felt ; for there are few plants which will' 

 not bear our highest summer heat. Accordingly, with us it is the 

 winter temperature, or rather the lowest temperature of the year, 

 which determines the existence of most perennials. We are all fami- 

 liar with this fact in the case of our ornamental greenhouse plants, 

 which are for the most part natives of warmer climates, and will 

 not bear the rigours of our own without protection ; and the same 

 thing is true of many native and acclimatized plants in seasons of 

 unusual severity. On the other hand, in the south-west of Ireland, 

 which has the highest winter temperature of any in the British 

 islands, there are twelve species of wild plants which are natives of 

 Spain. 



But within the limits beyond which the plant cannot live, there 

 are two other limits which determine its fruitfulness. It is now 

 well established that, in order to blossom, and to ripen its seed or 

 fruit, each species of plant requires a certain definite amount of 

 summer heat, above that which is necessary to the continuance of 

 its existence as a vegetable ; and that, provided it receives this 

 fixed amount, the time in which it is imparted is not essential. 

 This determines the lower limit to the successful culture of the 

 fruit-bearing plants. 



But there is also a higher limit, which is less obvious. Thus in 

 the case of the annuals, which include the plants most useful to 

 man, there is a certain temperature at which the plant becomes 

 perennial, and is propagated by lateral shoots, and not by seed. 

 When the temperature reaches this limit for any plant, its culti- 

 vation for seed becomes impossible, and we can use it only for its 

 leaves or root. On the other hand, tuberous plants become seed- 

 bearing, by the lowering of the temperature. We have a familiar 

 instance of the latter transformation in our fields. Most of our 

 green crops, such as turnips and mangolds, are cultivated for their 

 roots, and under ordinary circumstances produce neither flower nor 

 seed. But it sometimes happens that certain plants in a field have 

 received a smaller amount of heat than is required for this mode 



