FOLIAGE LEAVES. 43 



vine trails over irregularities in the surface, or climbs over and 

 through other vegetation. 



773. Position of leaflets on divided leaves. An interesting 

 comparison can be made with entire, lobed, divided and dis- 

 sected leaves. The entire leaf usually lies in one plane, since 

 usually the problem of adjustment is the same for the entire 

 surface. So the lobes of a leaf usually lie all in the same plane 

 as they would if the leaf were entire. We find the same is true 

 usually of the compound leaf. It forms an incomplete mosaic. 

 Some of the pieces having been removed allow much of the light 

 to pass through to leaves beneath. Leaves, especially those of 

 some size rarely lie in a flat plane. Some are more or less de- 

 pressed. Some curve downward. Compound leaves often 

 curve more or less and the leaflets often droop more or less in a 

 graceful fashion. It is interesting, however, that these far-sepa- 

 rated leaflets all lie in the same general plane. This is because 

 the area of the leaf, if not too large, makes the problem of posi- 

 tion with reference to light much the same as if the leaf were 

 entire. The leaflets or divisions, though separated, are laminate, 

 and they can work more efficiently facing the light. But suppose 

 we extend our observation to the finely dissected capillary leaves 

 of some of the parsley family (Umbelliferae), or to the upper 

 leaves of the fennel-leaved thoroughwort (Eupatorium foeni- 

 culaceum) among the aerial plants, and to Myriophyllum among 

 the aquatic plants. The divisions are threadlike or cylindrical. 

 One side of the leaflet is just as efficient when presented to the 

 light as another. As a result the leaflets are not arranged in 

 the same plane, but stand out in many directions. 



Occasionally one finds a divided or compound leaf in such a 

 position that one portion, because of being shaded above, receives 

 the stronger light stimulus from the side, while the other portion 

 is lighted from above. If this relation continues throughout 

 the growth-period of the leaf the leaflets of one portion may lie 

 in a different plane from those of the other portion. In such 

 cases, some of the leaflets are permanently twisted to bring them 

 into their proper light relation. 



