78 ELEMENTS OF GENERAL SCIENCE 



structure of the leaf, and we shall therefore study next this 

 organ, which seems to be the food factory of the world. 



85. Structure of leaves. The most conspicuous part of 

 the ordinary leaf is broad, thin, and green and is known as the 



blade. Besides the 

 blade there is usually 

 a stalk, called the 

 petiole, and some- 

 times also, at the 

 base of the petiole, 

 two little blades, 

 which are called stip- 

 ules. The petiole is 

 rather strong and 

 woody and serves to 

 hold the leaf blade 

 in the proper posi- 

 tion (fig. 40). 



The leaf contains 

 many veins, which 

 serve as a frame- 

 work to support it 

 as well as for other 

 purposes. In many 

 leaves there is a 

 main vein extend- 

 ing from the peti- 

 ole toward the tip 

 of the blade; smaller veins branch out from this and 

 extend obliquely toward the edge. Other leaves, like those 

 of the geranium or nasturtium, have no single central vein, 

 but, instead, several large veins extend outward like a fan 

 from the end of the petiole. There are other leaves, like that 

 of the common plantain, in which the principal veins all start 

 from the petiole and run nearly parallel toward the tip of the 



FIG. 40. Types of leaves 



A, leaf of the lady's-slipper orchid ; B, leaf of the 

 elm ; C, leaf of the maple ; J), leaf of the water shield 



