26 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



Phenologists have frequently endeavoured to estimate the accumu- 

 lated temperature that species presumably require for their various func- 

 tions. This reveals its existence most distinctly in spring, at which time 

 the opening of flowers and leaves is clearly dependent upon the conditions 

 prevailing in regard to heat and takes place in one year at one time, in 

 another year at another time, and in one place earlier than in another. 

 The number of days of vegetative activity commencing from a certain 

 date having been estimated, and the temperatures prevailing at various 

 places having been ascertained, endeavours have been made to explain 

 upon this basis the differences in development and the facts of distribu- 

 tion ; but in details there has been great diversity of treatment. Some 

 investigators have sought to estimate the accumulated temperature by 

 the addition of the daily mean temperatures ; others have multiplied 

 the mean temperature of a certain period (particularly the period of 

 growth) by the number of days ; others, again, have relied upon the 

 square of the mean temperature or of the number of days ; still others 

 opined that the daily maxima above o C, registered by a thermometer 

 exposed to the sun (insolation-maxima), should be added together. 

 These investigations absolutely demand the support of strictly scientific 

 experimental determinations of the temperatures important in relation 

 to the vital phenomena of the different species. But the results of these 

 estimations will not suffice to explain the extremely difficult and complex 

 question of the relations between heat and the distribution of species, or 

 between phenological phenomena, because other conditions, such as light, 

 the temperature of the soil, the after-effects of the preceding vegetative 

 season, are perhaps capable of replacing higher temperature. A general 

 source of error is that shade-temperatures, and not temperatures resulting 

 from insolation, are used in estimations; but even the sum of the insolation 

 temperatures would hardly give a correct account of the temperatures 

 prevailing during a definite period. 



In the following morphological features heat indubitably plays 

 a part : 



I. Many sub-glacial plants, particularly woody plants (Salix, Betula, 

 Juniperus, and others) assume the espalier-shape : that is to say, their 

 stems lie on the ground, are pressed against it, and concealed more or 

 less between other plants (mosses and lichens), stones, and such like ; 

 only their tips are directed upwards, sometimes almost at right angles, 

 and rise above the ground only a few centimetres. By this mode of 

 growth the plants doubtless receive a greater amount of heat than they 

 would were they erect ; but it is a question if it be not rather evaporation 

 resulting from the dry cold winds that induces this change of shape. 



The same form of growth is exhibited by many littoral plants (Atriplex, 

 Suaeda, Salicornia, Matricaria inodora, in Northern Europe ; Frankenia 

 pulverulenta, on the Mediterranean shores) ; it is not the lateral shoots 

 alone that lie prostrate and radiate in all directions ; but the main shoot 

 itself bends down, sometimes almost at right angles, prone on the soil.^ 

 Again this habit reveals itself on the desert and on sandy soil that is 

 strongly heated by the sun (e.g. Aizoon canariense, Cotula cinerea, and 

 Fagonia cretica, in Africa ; ^ Artemisia campestris and Herniaria glabra 

 in Northern Europe). In the hot dry chmate of the lowland of Madeira 

 ^ Warming, 1906. ^ Volkens, 1887. 



