104 ESSENTIALS OF BOTANY 



leaves. These differ in their nature and mode of origin 

 from the portions of the blade of a divided leaf. One 

 result of this diilerence appears in the fact that some time 

 before the whole leaf is ready to fall in autumn, the leaflets 

 of a compound leaf are seen to be jointed at their attach- 

 ments. In Fig. 67 the horse-chestnut leaf is shown at the 

 time of falling, with some of the leaflets already disjointed. 

 That a compound leaf, in spite of the joints of the sepa- 

 rate leaflets, is really only one leaf is shown: (1) by the 

 absence of buds in the axils of leaflets (see Fig. 68) ; (2) by 

 the horizontal arrangement of the blades of the leaflets, 

 without any twist in their individual leaf -stalks; (3) by 

 the fact that their arrangement on the midrib does not 

 follow any of the systems of leaf arrangement on the stem 

 (Sect. 12'4). If each leaflet of a compound leaf should 

 itself become compound, the result would be to produce a 

 twice compound leaf. Fig. 77 shows that of an acacia. 



122. Review Summary of Leaves."^ 



Parts of a model leaf J 



Classes of netted-veined leaves <j 



Classes of parallel-veined leaves ^ 



Relation of venation to number of cotyledons J 



Compound leaves; types dependent on arrangement of f 1. 



leaflets . | 2. 



Once, twice, or three times compound 



1 Illustrate by sketches if possible. 



