LEAVES AND THEIR USES 233 



or divided. So leaves may be twice divided, like those of the 

 bracken fern and the sensitive plant. Some ferns have 

 leaves three times divided, and the common meadow-rue has 

 a four-times-divided leaf. 



There are many other points of difference in the shape of 

 leaves ; some are circular in outline, some are heart-shaped, 

 some oval, some long and slender, and so on. These differ- 

 ences are important in the classification of seed plants, and 

 for this reason each particular form of leaf has been given a 

 special name. 



248. Leaves of Different Shapes on the Same Plant. 

 Not only are leaves of different shapes found on different 

 plants ; there are also differences in shape between leaves 

 borne on the same plant. Strictly speaking, perhaps, no 

 two leaves, even on the same plant, are ever of exactly the 

 same shape. But the differences in most cases are com- 

 paratively small, and the foliage leaves of a particular plant 

 usually follow the same general pattern. But there are 

 exceptions to this rule. The leaves of the common mul- 

 berry vary in shape to an unusual extent, even on the same 

 tree, some being deeply lobed, others heart-shaped and quite 

 without lobing. There are not a few cases in which different 

 parts of the same plant bear two very different kinds of 

 foliage leaves. One class of such cases is seen in some plants 

 the early internodes of -whose stem remain very short. The 

 result is the production of a cluster of leaves close together and 

 close to the surface of the ground. These are commonly, 

 but not properly, called " root leaves." Later the stem 

 forms long internodes, so that a tall, upright shoot is pro- 

 duced bearing scattered leaves. 



The root leaves and the upper leaves of the plant are often 

 decidedly different in shape, although if a complete series of 

 leaves is studied in the order of their age, a fairly gradual 

 change will usually be found from the form of the lowest 

 leaves to that of the uppermost ones. The common hare- 



