58 



A TEXTBOOK Ol-' BOTANY 



[Ch. Ill, 8 



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

 of which tliere are several kintls in adtlition to the more 

 famous one of our western pi'airies. Many other light ad- 

 justments are also kno\«i in natiu'e, not onlj' 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 thstinct functional 

 advantage to the organism can be clearlj^ 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. Thej' all 



perform the same function, and 

 their cUfferences correspond for 

 the most jiart 'i\-ith 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 conchtions 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. 2S. — The Banana, t^rowiiis 

 12 to 15 feet high, and Ijearing the 

 largest known simple leaves. (From 

 B.iifour, Class-bnok- of BuUinii.) 



