LEAVES 33 



52. The leaf-blade plane. — The blades of very few leaves of 

 the various fruits are flat, and there is much variation in the 

 varieties of any one species. Thus, in the apple, the blade of 

 Gravenstein is almost or quite flat ; saucer-shaped in the 

 Baldwin ; distinctly waved and more or less folded in Hubbards- 

 ton and Tolman; and reflexed or folded backward in Grimes. 

 Similar and other variations from a true plane are found in 

 the leaves of all of the fruits, some of 



which are valuable characters in iden- 

 tifying and classifying varieties. These 

 modifications of the plane are about as 

 constant as any other character of a va- 

 riety, but behave a little differently at 

 various ages, kinds of weather, and 

 even in different parts of the day. 

 Thus, in dry sunny weather folding 

 is more pronounced in most leaves ^ 



than in wet cloudy seasons, as it is ^"^^- "^condif Hcat^^^^^b' 

 also in the middle as compared with plaited; c. convolute. 



any other part of a sunny day. 



53. The arrangement of leaves on stems. — As noted in the 

 last chapter, buds and, therefore, leaves are arranged on stems in 

 a definite order which is constant for a species. Technically, the 

 attachment of leaves to the stem is spoken of as the insertion. 

 The mode of insertion of the leaves of all hardy fruits is alter- 

 nate or opposite and never whorled as in many other plants. 

 Buds open by the growth of leaves in regular succession on 

 stems, the youngest nearest the apex. When stems or buds are 

 injured, the leaves may not appear in their natural order. By 

 an orderly arrangement of leaves upon the stem, all become 

 equally well exposed to light and air. Therefore, when light and 

 air are much interfered with, the arrangement of leaves may 

 appear to be more or less irregular, a fact to be kept in mind 

 in studying leaves, especially of the grape where the disposition 

 of leaves, tendrils, and inflorescence mean much in classification. 



54. Vernation. — The disposition of the leaf-blade in the bud, 

 the manner in Avhich it is packed up, is called vernation. The 

 leaf-bud is commonly folded or rolled in the bud. "When the 

 blade is folded at the midrib so that the halves are face to face, 



