INTRODUCTION 31 



reason we have zones of vegetation depending on 

 climate, which is affected by heat (and moisture, 

 etc). In a similar manner altitude affects heat. 

 There is a fall in temperature as one ascends. The 

 zones of vegetation on mountains in the tropics in 

 this way are parallel with those encountered in 

 passing from the tropics to the poles. 



Plant life ceases near the zero or freezing-point on 

 a centigrade thermometer. But plants differ specifi- 

 cally in the amount of heat or cold they can with- 

 stand. And this especially applies to the most 

 important functions after those of nutrition and 

 growth, namely flowering and seed production. 

 The seed is provided against cold, and is the most 

 resistant to this factor. The duration of plants 

 reflects their adaptation to heat and cold, so we 

 have annuals, biennials, perennials, deciduous or 

 evergreen shrubs. The storing up of reserves in 

 bulbs, etc., is a modification to resist winter cold. 



Cold reduces absorption and causes physiological 

 drought. Similarly heat causes physical drought, or 

 also physiological drought, whilst it promotes the 

 plant's activities. 



4. Nutrition and Growth. 



Plants derive their food from two sources, generally 

 speaking, the air and the soil. Water-plants, as we 

 have seen, however, derive their food entirely from 

 the water, which serves for the two sources of food 



