648 



ECOLOGY 



diversity in branching is complicated and but vaguely understood. If 

 the terminal bud of the erect main shoot is removed, certain strong 

 lateral branches hitherto ascending obliquely soon begin to grow erect 

 (as in Picea), or buds hitherto inactive develop into erect shoots (as in 

 Araucaria). As in roots, the main axis seems to inhibit verticality in 



Fig. 955. — Alpine spruces (Picea) and firs (Abies), illustrating the spirelike contour 

 and excurrent habit that is characteristic of various conifers of high altitudes; Rocky 

 Mountains, Mont. — Photograph supplied by Elrod. 



the lateral branches. However, aerial stems differ strikingly from roots 

 in that light is an important factor in determining the orientation of 

 lateral branches; lateral stems, even those that descend, usually grow 

 in the direction of maximum incident light. 



The advantages of conical shape, elongation, and stem twisting. — 

 Other things being equal, the greater the power of stems to elongate and 



