VEGETABLE SPECIES. 23& 



which are more than usually sensitive to cold. The assiduity with 

 which botanists have considered this subject of a thermometrical mean 

 shows that more or less they have regarded it in the same light. It 

 has been the custom to attribute the disagreement between the limits 

 of plants and the lines of equal temperature to errors of observation on 

 the locality of species, to uncertainty respecting the thermometric 

 mean, or to causes acting independently of temperature, such as dry- 

 ness and humidity. But certain facts and the ingenious calculations of 

 M. Boussingault* on the heat requisite for culture in different countries, 

 have awakened some doubts as to the truth of this, and I have been 

 induced to attempt the solution of the question by a direct method. 

 The following is the course which I have pursued : 



I have so far investigated nearly forty species as to feel no doubt of 

 any importance with regard to their polar limits. These species have 

 been solely chosen with a view of avoiding causes of error, and of hav- 

 ing, at the same time, plants of different nature. Hence I have 

 selected species having their limit in Europe, inasmuch as Europe is 

 the only region where the local flora is numerous, and where the con- 

 ditions of temperature are well known. I have eliminated all the culti- 

 vated species, all species easy to confound with others, all which might 

 have escaped the observation of the authors of local floras, as well as 

 those in which synonyms might have occasioned embarrassment. I 

 have centred my researches on a dozen annual species, a dozen peren- 

 nial, and a dozen ligneous. I have established their polar limits by 

 means of a great number of floras and catalogues, by the inspection of 

 herbals, and also by questions addressed to botanists residing in 

 some of the less explored parts of Europe. I have succeeded in tracing 

 on the map the limit of these species. I have afterwards consulted 

 the most complete tables of the monthly temperature and seasons of 

 European cities, such as those of Kiimtz, Berghaus, Mahlmann, and 

 Dove, completing them by means of private researches. 



The following is what results at first view from this comparison, 

 founded on well ascertained facts : 



1. In no case does the limit of a species exactly coincide with a line 

 of equal temperature for any one period of the year. 



2. The limits of annual species, in the plains of Europe, cross one 

 another with considerable frequency. The limits of perennial and 

 ligneous species also cross each other in different directions,, and both 

 are far from being parallel when they do not thus cross. 



This single fact enables us to perceive how much the lines of vege- 

 tation differ from the lines of equal temperature ; for if we draw lines 

 founded on the equality of heat, at a certain season, they will be found 

 to vary little from parallel lines; if we take some other season such 

 lines will still appear nearly parallel with one another, though differ- 

 ent, without doubt, from the preceding. Thus, the isochimenal lines 

 will cross the isotheral ones, but they will never cross each other 

 at least in a level country. 



A little reflection will teach us that it would be chimerical to pur- 

 sue the comparison of vegetable limits with the lines of equal tempera- 



- Bouseingault, Economie RuraU, IT, p. 659. 



