GEOGRAPHY OF PLANTS. 513 



of hastening their period of maturity when judiciously planted, enable the 

 fanner to extend the cultivation over every district in which the summer tem- 

 perature reaches a certain point, however brief the duration of heat, provided 

 the definite aggregate adapted to each variety shall have been received. The 

 extraordinary high temperature experienced in certain northern localities re- 

 mote from the ocean, and the intense calorific and chemical action of the sun's 

 rays, enhanced by the extreme length of the days of summer, enable this plant 

 to mature in high northern latitudes. From the success that has attended 

 the growth of many varieties of maize, it is supposed that many other plants 

 may be acclimated, and that those whose epoch of growth would extend over a 

 period of low temperatures in northern localities may, like the maize, be suc- 

 cessfully carried towards the pole, if but new varieties adapted to the new con- 

 ditions could be produced. Such hopeful experimenters forget that the maize 

 is receiving on its further northern border the same, or nearly the same, aggre- 

 gate of heat it demands in its most favored clime, is exposed to the influence 

 of the same high mean during its most critical period, and, consequently, can 

 elaborate its juices and mature its seed as well in the shorter hot season of the 

 north as in the more temperate longer season of the south. 



The principle above enunciated is, however, subject to some modifications 

 as regards its application to certain physiological changes which have been 

 observed to result from long-continued efforts to cultivate plants under un- 

 natural conditions. Plants a'te, without doubt, capable of modification within 

 certain limits. Witness the numberless varieties of gi-ains and fruits, some of 

 which appear to be more hardy than their progenitors. Many of these which 

 appear to, or really do, thrive in more northern districts than tliose from which 

 they were derived, have merely acquired a greater susceptibility to the influ- 

 ences of light and heat, and are thence aroused into earlier action, quickened 

 in their vital functions, and mature under a lesser aggregate of heat as 

 measured by the thermometer, though for the critical periods of their existence 

 they demand the same mean temperature as the original from which they were 

 derived. 



It is in the power of man to fix these peculiarities when observed, and in a 

 measure to produce them by the selection of those which promise well, and 

 continuing the selection with adequate care through several generations. Vil- 

 morin, by skilfully applying the principles which influence plants in their ten- 

 dencies to sport into new varieties and directing them into the desired channel, 

 has almost created a new race of beets containing twice as much sugar as their 

 ancestors, and promising to be readily perpetuated.* 



Acclimating the tender plants of the tropics, and inuring them to the cold 

 seasons of the north or temperate latitudes, is, therefore, impossible, though, 

 some minor modifications upon those of short growth during the periods of 

 fervid heat of the northern summer have been made. The olive and the 

 orange have not been rendered more hardy, and the peach appears to be still 

 endowed with the same tenderness of bud it has always shown. 



The difficulties of acclimation may be illustrated by the fact that certain 

 vegetable products can be grown in particular latitudes, while others, though 

 they may attain considerable size, cannot be grown with any useful result. For 

 instance, in England the vine will never yield grapes capable of making wine 

 even of a quality equal to champagne ; nor will tobacco ever acquire that pecu- 

 liar principle which gives it, in the estimation of many, so great a value when 

 grown in some other countries ; and yet both the vine and the tobacco plant 

 flourish in the soil of England. The botanist and the meteorologist can 



* Silliman's Journal of Science, vol. XXII, p. 448, new series 

 39 A 



