THE FACTORS OF DISTRIBUTION 33 



escape low surface temperatures by their " geophytic " 

 habit, i.e. by dying down to underground and often 

 deeply -buried structures; whilst annuals, hibernating 

 or " perennating " in the seed stage, are only practically 

 affected by the temperature of the two to six months of 

 their vegetation-period. One of the chief dangers of 

 winter cold to perennials appears to be excessive tran- 

 spiration when the cold of the soil inhibits the action of 

 their roots. 



On the other hand, not only do Diatoms withstand 

 temperatures exceeding 80 C. (176 F.) in the waters of 

 hot springs, and saxicolous lichens 60-70 C. beneath 

 the cloudless summer skies in the desert; but flowering 

 plants, such as Rhododendron javanicum Benn., will 

 flourish amid the vapours of volcanic fumaroles; and 

 others, in India, when the shade temperature reaches 

 50 C. (122 F.), must be themselves heated to 60 or 

 70 C. 



The lowest temperatures at which seeds have been 

 observed to germinate range from below i C. for 

 Medicago satwa L., 1.8 C. for Flax, and 2 C. for many 

 Alpine species, to over 16 C. for Theobroma Cacao L. 

 Wheat will not sprout below 5 C. or above 37 C., while 

 the Vegetable Marrow will sprout at 42 C. The roots 

 of many Crucifercs will absorb water slightly above the 

 freezing-point; but those of the Tobacco' not below 

 12 C. Respiration has been detected in Juniperus at 

 - 10 C. and photosynthesis at -40 C.; whilst Arctic 

 Algae produce their reproductive structures in winter 

 when the water is not above 1 C., and may grow to 

 their full lengths when it is but little above o. Aquatic 

 plants in general are less exposed to rapid or extreme 

 fluctuations of temperature than are terrestrial species; 

 and, consequently, the minima and maxima of necessary 

 temperatures are nearer together in the case of the 

 former. In many plants of temperate or cold latitudes 

 it has been observed that a low temperature favours the 

 production of floral organs, a higher one that of vegeta- 

 tive shoots. 



Whilst only temperatures above a certain minimum 

 and below a certain maximum have any apparent effect 

 on plant-life or its various phases, it is rash to conclude 

 that the plant is not subsequently affected by those 

 outside these limits. Allowance must at least be made, 



