THE ANGIOSPERMAK : LK.WHS 



969 



be indicated for this peculiar change, which is habitual in the genus, but in 

 other cases the twisting of the leaf may have an ecological significance. Such 

 are, for example, the " compass plants " of the American prairies, tall 

 Composites {e.g., Lactuca scariola) which place their leaves pointing north 

 and south with the edges vertical (Fig. 958). In this way they receive equal 



Fig. 958. — Lactuca scariola. A compass plant showing the 

 north and south alignment of the leaves with thin edges 

 vertical. (Photograph taken in Chile.) 



lighting on both sides of the leaf, which has, correspondingly, an equifacial 

 structure, both sides being alike. Similar twisting movements may also occur 

 temporarily in plants exposed to very strong sunlight (Fig. 959), while in 

 Eucalyptus, although the twisting into the vertical position is permanent it 

 occurs only in the elongate mature leaves and not in the rounded juvenile 

 foliage. 



Leaf Folding. 



The young leaves which are still enclosed in a bud often show remarkable 

 foldings due to the tight packing imposed by limitations of space (Fig. 960). 

 This is not usually a mere crumpling of the young lamina, such as one might 



