416 Plant Physiology 



ment of the single shoot, of the plant as a whole, or of a 

 group of plants is of the same nature, ample considera- 

 tion being given to other forces which may be operative 

 at the same time. Trees thickly branched, such as the 

 Norway maple and the linden, or the unpruned apple and 

 pear, will exhibit in a significant manner this effect, ex- 

 posing a complete shell of leaves. (See Chapter XX for 

 growth movements.) 



When trees grow up close together in the forest the lower 

 branches are ultimately too much shaded, so that these are 

 killed and in time drop off. The leafy shoots are confined 

 to the uppermost parts, and this system of constant self- 

 pruning through the survival of those favorably placed 

 results in the characteristic long trunks of the forest trees 

 as compared with the shorter trunks and abundant 

 branches of isolated specimens in the lawn or meadow. 

 The tall trunks are, of course, most desirable from the 

 standpoint of the lumberman; but, at the same time, the 

 decayed branches or stumps offer favorable opportunity 

 for the entrance of destructive fungi which cause great 

 annual loss through the decay of sap or heart wood, and 

 thus artificial pruning possesses great advantages. 



252. Light perception. - - Phototropic organs may pos- 

 sess special perception regions, and these regions do not 

 necessarily correspond to those of curvature or bending. 

 The method of perception is not understood, but the sensi- 

 tiveness of the mechanism is almost incredible. 



The perceptive mechanism resulting in leaf orientation 

 has received much attention. Haberlandt and others find 

 in the lens-shaped cells and cuticular thickenings of epi- 

 dermal cells the structures which they regard as indirectly 



