34 PLANT GEOGRAPHY 



in considering the effects of Spring sunshine, for heat 

 received by perennial plants in the previous season, and 

 for heat in the soil and its water-content. A little con- 

 sideration of the methods of the horticulturist will 

 remind us that he not only regulates the supply of heat 

 to the air round his plants at various seasons, but also 

 controls their supply of light, moisture, and bottom-heat. 



It appears immaterial, within surprisingly wide limits, 

 whether a plant receives heat in a short or a long period 

 of time. In northern India, for example, Wheat is 

 harvested within three months of being sown; in 

 Palestine within five months; in Sicily in six; in 

 Central Europe in nine; in Yorkshire in eleven, and in 

 Scotland in thirteen. At the same time, the duration 

 of the period when useful temperatures render active 

 plant-life possible produces marked effects upon the 

 physiognomy of vegetation. Annual species are rare in 

 Arctic latitudes and alpine altitudes, because the period 

 in which useful temperatures exist is too short for their 

 life-cycle ; whilst our early-flowering perennial herbs de- 

 pend on the stored-up food and energy of the preceding 

 autumn ; and the dense evergreen vegetation of equatorial 

 regions is correlated with an almost complete absence of 

 seasonal change, heat and moisture prevailing throughout 

 the year, and vegetation, in consequence, undergoing no 

 period of rest. 



The distributional limits of species seem, however, 

 to be rather determined by the extremes of cold and 

 heat to which they are subjected than by annual mean 

 temperatures. Those that are checked from extending 

 polewards by the want of sufficient heat, such as many 

 annuals, are termed philotherms , or heat-lovers; whilst 

 those that are checked by too great cold, such as most 

 evergreens, are known as frigofuges, or cold-fearers. 

 Owing to the rarity of frosts in insular climates frigofuges 

 can spread more polewards in them; while philotherms 

 expand more readily in continental regions, often passing 

 through the unfavourable extreme of climate either in 

 the seed stage or as geophytes, i.e. underground. 



It is difficult to correlate definitely many external 

 characteristics of form in plants with heat ; but it seems 

 that the prostrate or " espalier " habit of woody plants 

 just below the snow-line, such as species of Willow, 

 Birch, and Juniper, and the development of rosettes of 



