OF PLANTS. 265 



tainty in the estimate of the days, the accurate mean 

 temperature and the want of knowledge whether or not 

 the same kind of Barley is cultivated in all the places, will 

 allow us to expect Similar results are obtained for Wheat, 

 Maize, the Potato and other cultivated plants. We may 

 express these results thus : Every cultivated plant requires 

 a certain quantity of heat for its development, but it is the 

 same thing whether this heat is distributed over a shorter 

 or longer space of time, so that certain limits are not 

 exceeded ; for where the mean temperature sinks below 

 36 24', or where it rises above 71 36', Barley will no 

 longer ripen. Consequently, to define accurately the con- 

 ditions of temperature which a plant requires, to maintain 

 it in a flourishing condition, we must state within what 

 limits its period of vegetation may vary, and what quantity 

 of heat it requires. This most remarkable circumstance 

 was first observed by Boussingault, but unfortunately, we 

 as yet possess not nearly sufficiently accurate accounts of 

 the conditions of culture, in the various regions of the 

 earth, to enable us to follow out this ingenious view in all 

 its details. 



I have chosen the Barley as an example, in the preceding 

 remarks, because it has the widest range of distribution of 

 all the Cerealia, and is cultivated from the extreme limits 

 of culture in Lapland, to the heights immediately beneath 

 the equator. But it has by no means the same importance 

 everywhere that it has in the northern region, where, in a 

 little narrow zone, it appears as the sole bread-corn ; and 

 in the following observations on the distribution of the 

 more important Cerealia, it will be considered only in 

 reference to this last point. In Lapland and northern 

 Asia, Rye soon appears beside it, but by the inclemency of 

 the climate confined to favourable years, and therefore not 



