58 



A TEXTBOOK OF BOTANY [Ch. Ill, 1 8 



toward the midday sun, produces the " Compass plants," 

 of which there are several kinds in addition to the more 

 famous one of our western prairies. Many other light ad- 

 justments are also known in nature, not only in leaves and 

 stems, but also in roots, flowers, and other parts. They 

 include movements towards, from, and variously across the 

 line of incident light. In many cases, a distinct functional 

 advantage to the organism can be clearly perceived, but in 

 others this is not evident, though here the limitations of our 

 knowledge may be at fault. 



8. The Various Forms of Foliage Leaves 



Foliage leaves are remarkably diverse in their sizes and 

 shapes, despite their singularly uniform thickness. They all 



perform the same function, and 

 their differences correspond for 

 the most part with differences 

 in the habits of the plants 

 which produce them. 



The sizes of foliage leaves 

 range all the way from almost 

 microscopic up to that of 

 Palms and Bananas, several 

 square feet in area (Fig. 28). 

 Marshaling sizes against habits 

 we find in general that the 

 largest leaves occur upon 

 plants which have the most 

 abundant water and warmth, 

 and least exposure to bright 

 sun and winds, — in other 

 words, upon plants exposed to 

 relatively least transpiration. 

 These conditions are best realized in the shelter of tropical 

 forests, and there we find the largest leaves, as all pictures 

 of tropical undergrowth well show (Fig. 29), while the same 



Fig. 28. — The Banana, growing 

 12 to 15 feet high, and bearing the 

 largest known simple leaves. (From 

 Balfour, Class-book of Botany.) 



