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 difference 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. 124). If each leaflet of a compound leaf should 
itself become compound, the result would be to produce a 
twice compound leaf. Fig. TT shows that of an acacia. 
122, Review Summary of Leaves. 
1. 
Parts ofa modelleaf. . . . . 2... 1... we 7 oe 
3. 
Classes of netted-veined leaves . { 4 
Classes of parallel-veined leaves . . . . . 2... { - 
Relation of venation to number of cotyledons. . . . . { 
Compound leaves; types dependent on arrangement of { 1. 
leaflets . i: 2. 
Once, twice, or three times compound { 
1 Tllustrate by sketches if possible. 
