METABOLISM 47 



this is avoided in various ways. Under 

 arid conditions evaporation from the 

 surface of the leaf is often checked by a 

 covering of hairs (giving a gray aspect 

 to desert plants) or by wax, resin or var- 

 nish. The leaf surface is often greatly 

 reduced and the stomata, or openings in 

 the surface of the leaf by which water 

 escapes, are fewer in number. In moist 

 and shady situations the opposite tend- 

 ency is shown: leaves become larger, 

 smoother, thinner and have more sto- 

 mata. 



The arrangement of the leaves on the 

 stem follows a regular order which can 

 be expressed mathematically. This was 

 at first interpreted mystically but is now 

 believed to be due in many cases to mutual 

 pressure of the young leaves at the tip of 

 the stem. This illustrates how theories of 

 development often begin in an atmos- 

 phere of mysticism but end with a purely 

 mechanistic explanation. 



This arrangement, which often in- 

 sures a fairly equal exposure of leaves to 



