400 THE CLIMATE OF IRELAND, AND 



of growth ; and these plants throw tip a lofty stem, and flower, 

 while the tuberous growth ceases altogether. This is one of those 

 many beautiful adaptations which compel us to look up from 

 nature to nature's Grod. When the plant is under such conditions 

 that it must perish, it sends up its spikelet of flowers as it were 

 by an instinct and matures the seed, which is destined to continue 

 the life of the species, while the individual fades and dies ! 



And here I cannot refrain from throwing out a suggestion 

 which, if found to be true, may be of some practical importance. 

 The plant which we chiefly cultivate for its root, and whose life 

 and health is of such importance to this part of the kingdom, pro- 

 duces (as you know) flower and seed, as well as tuberous root. I 

 venture to suggest to physiologists the inquiry, whether this cir- 

 cumstance may not be closely connected with the uncertainty of 

 the crop ? and whether, if means could be found to repress the 

 flower, the root might not acquire vigour and strength to resist 

 disease ? But however this may be, the transition from the con- 

 dition of perennial to that of annual is a subject of much import- 

 ance in connexion with the question of the acclimatization of plants. 

 It is probable that there are many tropical plants, unknown to this 

 climate, which are perennial in their native place, but which would 

 become annuals, with seed and fruit, when transferred to a colder 

 clime. When we consider the acquisitions which have been already 

 made by such transfers, we can hardly over-estimate the import- 

 ance of future gains, or the benefits which may be thus acquired 

 for the human family. 



But the lower limit of summer temperature is by much the most 

 important to us ; for it is that which in our climates determines 

 the question of success in the culture of the cereals. I will there- 

 fore refer to it a little more particularly. 



There is no doubt that the integral of solar heat, taken between 

 the limits of time when the vegetation becomes active in Spring, 

 and when the seed is ripened in Autumn, is the element which 

 determines one of the geographical limits which Nature has im- 

 posed to the culture of the plant. To find this, for each important 

 species of plant, would require an amount of observation which 

 has not yet been bestowed. In the meantime, however, a tolerable 

 approximation to its value may be made, by taking simply the 

 mean summer temperature i.e., the average temperature of the 



