EXTERNAL CHARACTERISTICS 



2IS 



in mind that we have no evidence that cold conditions 

 produce hairs or leaf coatings ; we simply note that plants 

 having such structures appear to do better in such regions 

 than plants which lack them. 



Cutin, which is present in the older epidermal cells of 

 roots, is also present in the epidermal cells of leaves. Just 

 as it prevents absorption in roots, it prevents evaporation 

 in leaves. Sometimes it is present in such abundance 

 as to give a characteristic appearance and texture to 

 leaves. Probably you have noticed how stiff the leaves 

 of Christmas holly are; this stiffness is due to an abun- 

 dance of cutin. 



E. Leaf Size 

 and Foliage Area. 

 — Leaves are of 

 all sizes ; they 

 range from the 

 tiny leaves of a 

 moss, which may 

 be no larger than 

 the head of a 

 pin, to the huge, 

 round leaves of 

 the Victoria re- 

 gia, a tropical 

 water lily, whose 

 leaves may be 

 seven feet in 

 diameter. (See 

 Figure TJ A.) 

 The largest 



Fig. ^^A. — Leaves of a tropical water lily {yictoria 

 regia), growing in Lincoln Park, Chicago. 



