32 PLANT GEOGRAPHY 



Tropic (Summer and Winter). As a mean temperature 

 may be a mean between extremes differing much or 

 little, it is important from the point of view of plant-life 

 to trace the isotherms of greatest heat and greatest cold. 

 It is more usual to take those of the hottest and coldest 

 months (July and January), which are known as 

 isotheres and isocheims. Whilst in the South Temperate 

 Zone, in which there is a continuous ocean belt, the 

 isotherms are nearly parallel, in the Northern Hemisphere 

 they are generally deflected polewards during the 

 summer as they traverse the continents, and in a reverse . 

 direction in winter, or when traversing the ocean. 

 Ocean-currents, conveying large volumes of water, 

 heated in equatorial, or cooled in polar, regions, modify 

 the positions of isotherms. 



Most of the water-vapour in the atmosphere being in 

 its lower portion, high ground with drier air radiates 

 heat more than does low ground, and thus has lower 

 air-temperatures. Temperature falls about i F. for 

 every 300 feet of altitude. Soils, too, differ both in the 

 amount of heat they reflect, owing to then: light or dark 

 colour, and in their retention of heat, according to their 

 moisture and porosity. Dry soils with much interstitial 

 air, such as sand, carry off heat more slowly than damp, 

 close-textured soils, such as clay, and as a consequence 

 both become hotter themselves and maintain a higher 

 air- temperature overhead. 



The requirements of a plant as to heat vary, not only 

 according to its species, but for each of the various 

 stages and processes in its life. For each of these, as 

 well as for the existence of the plant, there are three 

 cardinal points or degrees of temperature : the minimum 

 or lower critical temperature, or lower zero-point, the 

 optimum, and the maximum, upper critical temperature, 

 or upper zero-point. Nowhere on the earth's surface 

 does the temperature appear to be too low or too high 

 for some plant-life. At Yakutsk, where the tempera- 

 ture falls to - 62 C. (below - 79 F.), 200 species survive, 

 some even hibernating when in flower-bud. Seeds, and 

 other parts of plants containing but small proportions 

 of water, are capable of resisting intense and prolonged 

 cold, the destructive action of cold being apparently 

 either the rupture of tissues by the formation of ice 

 within the cells, or drought. Herbaceous perennials 



