210 



STRUCTURE AND VITAL PROCESSES OF PLANTS 



forests. In each plant an invisible stream of water rises, as it 

 were, from the ground to increase the ocean of the atmosphere, 

 afterwards to come down again to the earth as rain. 



The amount of evaporation varies with certain circumstances. 

 In the first place, it depends on the temperature: the warmer 

 the air is and the hotter the leaves become under the rays of the 

 sun, the more rapid will be the action of transpiration. Secondly, 

 when the wind blows and carries away the air round the leaves, 

 which are saturated with vapour, bringing dry and 

 thirsty air ready to take in vapour from them, 

 the amount of evaporation will naturally be 

 greater than when there is no wind. Thirdly, 

 clothes dry much more slowly during the monsoon 

 tlian when the dry land-wind blows. For the 

 same reason plants will evaporate much more 

 water in dry weather than in the monsoon. When 

 the air is saturated with moisture, e. g., in the 

 cool morning of a monsoon day, no water can 

 be evaporated from the leaves. Some plants, as 

 Taro (Colocasia), ^laize. Bamboo, are able to 

 press the water out from little pores at the tips 

 and edges of their leaves. These drops of water 

 hanging from the tips of the leaves should not be mistaken for 

 dew-drops. 



(e) Helps to promote Transpiration. — As the evaporation of 

 water through the pores of the leaves is of such great importance 

 to the plants, we find many arrangements in the plant-world by 

 which this process is enhanced wlion likely to prove useful to 

 the plant. 



Plants that grow on shady and moist places have lan/e leaves, 

 as a rule, and generally very numerous stomata. Their leaves 

 are, besides, very tender, that is to say, their epidermis is so thin 

 that water can pass not only through tlie pores (stomata), but 

 also throufjh the ivalls of tlie surface cells. 



We can sometimes see dark spots or blotches on the leaves 

 of some plants, such as some Aroideo} or Turmeric. By virtue of 

 these dark-coloured spots the leaves are enabled to absorb more heat 



Fig. 194. — A drop 



of water oozing 



from the leaf of 



Colocasia. 



