136 The PlaxNT World. 



THE INFLUENCE OE LOW TEMERATUREvS ON THE 

 DISTRIBUTION OF THE CxIANT CACTUS. 



Forrest Siikkve. 



'i'lic work of ]\Iuller-Thurgaii and of Molisch has shown the 

 variety of w a\s in which low temperatures may be fatal to dif- ' 

 fcrcnt species of plants. Tropical forms may be killed by con- 

 tinual exix:)sure to temperatures just above freezing; other 

 forms sur^•ive temperatures slightly below freezing, but suc- 

 cumli (ui tlie formation of ice in tlieir tissues. Certain plants will 

 withstand the formation of ice in their intercellular spaces at 

 a few degrees below freezing, but will die at a sudden pronounced 

 lowering of temperature while tlle^' are in a frozen state; at the 

 same time that very luany arctic species will withstand total 

 freezing together with ver>' low temperatures. In many cases 

 that have been investigated a sudden thawing proves fatal 

 after a given set of cold conditions, although a gradual raising 

 of temjierature enables them to survive. 



The low temperatures of winter have been recognized for 

 a long time, both in scientihc and practical work, as important 

 factors in limiting, the northward and vertical distribution of 

 tropical and sub-tropical i^lants. The fact that these plants 

 varv greatl\- among themselves in resistance to cold, and the 

 fact that it is different ])hases of winter cold that are fatal to 

 them, is i)ro\cd 1)\- the lack of a coincidence in the northern 

 limit of distribution among an\- considerable number of them. 

 The line which marks the extreme southern limit of frost is 

 the most important climatic boundary in restricting the north- 

 ward cxtensi'Mi of ])eremiial trop.ical s])ecies, and it is the line 

 along which the inlhience of winter cold is the simplest in its 

 operati n. In the huiitation of the sub-tropical species which 

 extend nortjiward of tliis line a \ariety of phases of winter cold 

 \\illiont (loul)t are o] eratixe. Chief among these are: the 

 greatest number of consecutive hours during which the temj'jera- 

 ture falls below freezing; the total number of hoius of frost in a 

 single winter; the absolute mininnnn reached and the length 

 of the winter, reckoned from the first frost of autumn to the 



