Laws of Temperature Control of the Geographic Distribution of Terrestrial Animals and Plants

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publisher not identified, 1894 - Biogeography - 10 pages
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Page 238 - ... directed attention to one of the physiological effects of a lowering of temperature, namely, its influence upon diminished or increased fertility and the rate of reproduction in what he has called the 'law of temperature control'. This he has stated as follows : temperature by controlling reproduction predetermines the possibilities of distribution; it fixes the limits beyond which species cannot pass; it defines broad transcontinental barriers within which certain forms may thrive if other conditions...
Page 234 - ... are restricted in southward distribution by the mean temperature of a brief period covering the hottest part of the year.
Page 233 - Animals and plants are restricted in northward distribution by the total quantity of heat during the season of growth and reproduction.
Page 236 - F. ). The northern boundary of the Lower Austral zone, therefore, is marked by the isotherm showing a sum of normal positive temperatures of 10,000 C.
Page 231 - Physiological botanists have long maintained that " the various events in the life of plants, as leafing, flowering and maturing of fruit, take place when the plant has been exposed to a definite quantity of heat, which quantity is the sum total of the daily temperatures above a minimum assumed to be necessary for functional activity.
Page 233 - ... 6- C. in spring and continuing until it falls to the same point at the end of the season. The sums thus obtained were platted on a large scale map of the United States and connected by isotherms, which were found to conform in the most gratifying manner with the northern boundaries of the several life zones.
Page 232 - In conformity with the usage of botanists, a minimum temperature of 6 C. (43 F.) has been assumed as marking the inception of the period of physiological activity in plants and of reproductive activity in animals
Page 237 - F.); and since the tropical life region is a broad equatorial belt, it is probable that both its northern and southern boundaries are marked by the isotherm showing a sum of normal positive temperatures of 14,000 C.
Page 238 - ... ...Humidity and other secondary causes determine the presence or absence of particular species in particular localities within their appropriate zones, but temperature predetermines the possibilities of distribution ; it fixes the limits beyond which species cannot pass ; it defines broad trans-continental belts within which certain forms may thrive if other conditions permit, but outside of which they cannot exist, be the other conditions never so favourable.
Page 232 - If it is true that the same stage of vegetation is attained in different years when the sum of the mean daily temperatures reaches the same value, it is obvious that the physiological constant of a species must he the total quantity of heat or sum of positive temperatures required by that species to complete its cycle of development and reproduction.

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