1837*3 Extra-Tropical Plants within the Tropics* 



29£ 



of all the kinds and varieties of European corn subjected to experiment,- 

 was prevented by the high temperature of July (71° F.); but that, on the 

 heat afterwards subsiding in August (to what degree is not' mentioned), 

 they all shot out fresh erect stems, and many of them produced ears, 

 though, owing to the advanced season of the year, none matured its 

 crop. It was also observed that the winter wheat died down to the 

 root in winter, but not the root itself, and on the return of spring pro- 

 duced a luxuriant crop, which ripened in the usual way. In India 

 we sow in October and November to reap in February and March, 

 (Victoria wheat is quicker, requiring only about 100 or 110 days 

 for maturation) : the mean temperature for these months is for 

 October 82°, November 78° 9', December 76°, January 75.5, Fe- 

 bruary 77° 7'« Supposing the seed sown in October, after the 

 setting in of the rains, under a temperature of 82° (11 degrees 

 higher than that of Paris in July), the progressive diminution of heat,- 

 during its after stages, enables it to advance uninterruptedly until the 

 increasing heat of February brings it to maturity ; whereas, if sown in 

 December or January, when the heat is at its minimum, the result of 

 the Paris experiment gives us every reason to suppose that the rise of 

 temperature in February would arrest its progress, in the same way as 

 happened in France ; and there is equal reason to infer, if the failure in 

 Xalapa is justly attributed to the heat, that the adoption of a similar 

 modification of the method of culture would enable them to raise grain 

 in place of merely pasture crops, and at the same time supply them- 

 selves with «, variety of grain suited for successful culture in their, 

 climate. This view, which seems fully in accordance with the 

 experience acquired in tropical climates, is at variance with some- 

 points of that obtained in temperate ones. Thus it is recorded in 

 the memoir that winter wheat, sown in March, when the temperature- 

 was 6 degs. 5m. centigrade (about 44° Fah., a temperature too low for 

 germination), which did not germinate till the heat had risen & degrees, 

 yet continued, notwithstanding a rapidly increasing temperature, stea- 

 dily to advance to maturity. These cases seem so diametrically opposed 

 as to be inexplicable on any known principle, and can only be account- 

 ed for on the supposition that art can change the course of nature, in so 

 far at least as is manifested in these vital operations of plants. Accord- 

 ing to the laws of vitality explained in my former communication, we 

 might perhaps say that the winter variety requires a very high compa- 

 rative degree of heat, to supply which it is necessary, in the first instance 

 to increase its susceptibility for that stimulus by exposure to cold ; and 

 that the other has had its constitution so modified that the opposite con- 

 dition has been established, and that it, therefore, requires to have its 

 susceptibili ty diminished by previous exposure to a higher temperature 



