546 



ECOLOGY 



strikingly with the sparse undergrowth of dense shady woods of beech or hemlock, 

 the plants in the latter consisting largely of thin-leaved mosses, ferns, and other 

 shade plants (rig. 784). In early spring our deciduous forests are well lighted, 

 and the undergrowth then displays remarkable activity; while in many plants the 

 leaves remain alive through the summer, in others they soon die (as in Claytonia, 

 Dicentra, and Allium tricoccum). The great intensity of tropical light often permits 



FIG. 784. A plant of wild spikenard (Aralia racemosa), displaying a kind of leafage 

 common in rich mesophytic woods; note the large, compound diaphototropic leaves 

 with broad leaflets, which are very thin and capable of enduring considerable shade; 

 Manitou Island, Michigan. Photograph supplied by THOMPSON. 



a dense undergrowth in the forest shade; where evergreens prevail, the herbage 

 always is exempt from direct insolation. 



Vertical leaves. Some leaves (especially among xerophytes) are 

 slightly if at all diaphototropic, assuming a vertical position through 

 their growth activity. Lactuca scariola, for example, has diaphototropic 

 leaves in the shade, but in the sunlight the leaves twist about into the 

 profile position (fig. 785). In the compass plant (Silphium laciniatuni) 

 and often in Lactuca the leaves not only are vertical, but also face 

 east or west. In Eucalyptus globulus intense light induces not only a 

 vertical instead of a horizontal position, but a change in leaf form as 

 well. Such changes in reaction accompanying an increase of light in- 



