36 



THE FACTORS 



[Part I 



follows 1 : ' Most kinds of moss growing here [Spitzbergen] are forms which are 

 more or less imperfect and injured by frost. In fact, they suffer greatly from the 

 rigour of the climate, for the whole plant usually assumes a dark tint ; the stems 

 become shorter, more richly branched and more caespitose than usual ; the leaves 

 also are modified in shape and direction of growth, and are more crowded, shorter, 

 less pointed, and more erect or adpressed and concave ; in addition they are 

 frequently white or transparent at the tip, because the chlorophyll is frozen; if in 

 the well-developed plant the midrib of the leaf be continued into a long hair-like 

 termination, here it may be that it seldom protrudes beyond the leaf-tip.' Similar 

 observations were made by Berggren 2 : 'As regards mosses, the characteristics are, 

 that the leaves are broader, very often concave, and have a tendency to form hood- 

 like tips. ... It is quite the exception to find mosses that are distributed from the 

 temperate zone up as far as Spitzbergen, which have not shorter and, in consequence, 

 relatively broad leaves. . . . Sometimes the margin of the 

 leaf is bent back as well, and its teeth disappear.' 



Kjellman gives similar results for several plants of 

 a higher order, and refers to the cones of Picea excelsa 

 and the leaves of a few Ericaceae. There is, he says, 

 the same tendency as in mosses for the leaves to become 

 broader and shorter and to have any irregularity in their 

 margin removed. 



Further research must decide how far these modifica- 

 tions depend directly on temperature. 



Warming 3 observed in Juniperus communis (Fig. 36), 

 as well as in Lycopodium annotinum and L. Selago, 

 a tendency to bear leaves which are straighter and 

 adpressed and do not stand off from the stem as is 

 elsewhere the case. He considered this to be a method 

 of protection against transpiration. During winter many 

 species of pines resemble such ' cold forms,' as their 

 needles become raised and pressed against the stem. 

 I do not know of any such effects produced by drought \ 



Fig. 36. Juniperus com- 

 munis. A The form nana 

 of a cold climate. B The 

 common form. After Warm- 

 ing. 



Every plant can live only at temperatures lying between two extreme 

 degrees, which are more or less far apart, and are termed respectively its 

 upper and lower zero points. The overstepping of either of these limits, 

 sooner or later, but at the latest within two or three days, results in the 

 death of the plant. The zero points vary for different species ; on the 

 other hand, individual plants of one species, provided they have grown in 

 nearly similar environments, have the same zero points. The absolute 

 extremes of plant-life are not identical with those of all its functions. Each 

 function has its own extremes, and at a certain degree of temperature its 

 optimum. There are, then, three cardinal points or degrees. Like the 



1 Lindberg, op. cit. p. 536. 



3 Warming, op. cit. p. 114 and Figs. 12-14. 



2 Berggren, op. cit. p. 17. 

 1 Johovv, op. cit. 



