Page:Journal of botany, British and foreign, Volume 9 (1871).djvu/405

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Botanical Society of Edin burgh. — November 2th. — Alexander Bucliaii, M.A., President, iti the chair. The President delivered an in- augural address, of which the following is an abstract : —

I propose in this address to make some remarks on climate and wea- ther relative to the geographical distribution of plants, havini>- recently completed certain inquiries regarding prevailing winds and rainfall which are so intimately connected with the subject. In 1847, De Can telle made some researches into the causes which limit vegetable species t )wards the north in Europe and similar regions, and arrived at the conclusion that every species having its northern limit in central or northern Europe ad- vances as tar as it finds a certain fixed amount of heat, calculated from that day of the year when " a certain mean temperature commences to the day when that mean terminates." This law he applied with great inge- nuity, in explanation of the northern limit of Ali/ssiim calyclnaai, Euohij- mus europcsan, and Blantlius Carihnsianornm. It is evident that this law can be best tested by applying it to the limits of wheat, barley, the vine, and other cultivated species, since owing to the care taken of them by man in their cultivation, it' may be consideretl that it is climalic condi- tions alone which set the limits to their distribution. According to Boussingault, wheat requires 8248° F. from the time it begins to grow in spring for the proper ripening of the crop.; and moreover, this heat must be so distributed as to secure a mean temperature of 58° during the period when the seed is ripening. This statement referred to the conti- nent of Europe, to which his researches were confined. But we have found in Scotland that a mean temperature of 56° with the average amount of rain and sunshine, is sufficient to ripen wheat properly. Not only so, but the crops of 1864 ripened well with a temperature as low as 54"4 ; in this year, however, the sunshine was much above the average, and the mean of the day temperatures was high. Now whatever be the cause, whether the longer day in Scotland, or the clear sky, or bjth com- bined, which brings about the above results, it is clear that in co isidering the infiuence of temperature on the ripening of plants, it is not merelv mean temperature, but the manner in which this vital element is par- titioned during the twenty-four hours which must be taken into aci'ount. Now what most influences the mode in which temperature is dislribntcd during the day is the amount of cloud and moisture in the atmosphere, to a knowledge of which the rainfall through the months of the year may be regarded as furnishing the best available key. Indeixl, so great is the direct and indirect infiuence of moisture on plants, that we shall not be far wrong in supposing it to be co-ordinate with that of temp 'ratiire. De Candolle's researches applied to a region where the climates are de- termined rather by variations of temperature than of moisture. Not the least valuable of the results he arrived at in ap])lying the law to other regions is this, viz. " On the borders of the Mediterranean St^a, the limits appeared so often determined by the humidity, or by causes still unknown, that the operations of temperature always eluded my calculations." In the climates of this region the rainfall plays a conspicuous part, and hence, as was to have been expected, an assumed climatic limit to species which did not include the rainfall, turned out to be inapplicable to the facts of distribution. Again, perennial species, especially trees and shrubs, are in many cases arrested, or limited, by the absolutely lowest tempera- tures that occur. The great frost of Christmas 1860 brought us very

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