2i8 LEAVES 



ditions to which they seem to be well suited. It is well 

 to bear in mind the principle that structures must appear 

 before their suitableness can be determined; it is no 

 longer believed that the need for a certain structure is a 

 cause of its appearance, though of course the advantage 

 or disadvantage of a structure to a plant may determine 

 whether it continues to exist or not. It is believed that 

 some structures which plants possess are of no advantage 

 to them at all. Similarly, in our own bodies, the vermi- 

 form appendix is a structure which seems now to be 

 more of a disadvantage than an advantage. 



By foliage area we mean the area of all the leaves of a 

 plant. In connection with foHage area, root area should 

 be in mind. Evidently a plant is limited as to the amount 

 of its leaf area by the amount of its absorptive area, its 

 root-hair area. It cannot maintain more leaves than it 

 has root-hairs to supply them with water. Foliage area 

 must be limited also by the amount of water in the soil. 

 In dry soil more absorptive area is required to maintain a 

 given amount of foHage area than in moist soil. Thus in 

 desert plants the root area is usually much larger in pro- 

 portion to the leaf area than in plants which grow where 

 moisture is more abundant. ' 



Large foHage area is evidently an advantage so far as 

 photosynthesis is concerned, but a disadvantage so far as 

 the danger of drying out is concerned; small leaf area is 

 good for protection, but poor for food making. These two 

 things are checks on each other, and you will find that in 

 many other features than in leaf area the plant seems to 

 preserve a balance between photosynthesis and protec- 

 tion. Too much of one is sure to result in too little of 

 the other. 



