74 NATURE TEACHING 



thus further reducing evaporation. Many leaves have 

 their pores so placed that when the leaf is curled up 

 during dry weather they are all under cover, none 

 being present on the exposed outer side. Thus it will 

 be seen that order prevails even under such disturbing 

 conditions as those which lead to the withering of 

 leaves by drought, when all appears confusion. There 

 are many other contrivances for protecting plants from 

 excessive loss of water. Amongst the most common 

 are the thickening of the outer skin, well seen in house- 

 leek, laurels, box, etc., the provision of a coating of hair, 

 for instance, in the mullein, and by the reduction in size 

 of the leaves, as, for example, in the pines, heaths, furze, 

 and other plants which can live in situations where they 

 get but little moisture. 



It is important that the pores in the leaf should be 

 enabled to perform their functions under all the 

 conditions to which the plant may be exposed." We 

 have already seen how in some plants they are covered 

 and protected during drought. It is also often essential 

 that they should not be readily filled by drops of water 

 during rain or dew, and the surfaces of leaves often 

 have slightly waxy or hairy coatings, so arranged that 

 those parts of the leaf which are abundantly provided 

 with pores are extremely difficult to wet, while surfaces 

 with few pores are wetted easily. Good instances of 

 this are seen in leaves which are easily wetted on the 

 upper surface where there are no pores, but which 

 throw off water from their under surfaces in a wonderful 

 manner. The leaves of water-lilies, duck-weed, frog- 

 bit, etc., cannot be wetted on their upper surfaces, 



