CHAPTER XI. 



PHYSIOLOGY OF THE LEAF MOVEMENTS AND 

 POSITIONS OF LEAVES. 



Principal function of the leaf Photo-synthesis Position of leaf- 

 Curvatures and movements of leaves Geo tropic, heliotropic 

 and sleep-movements Mechanism Nutations and periodic 

 movements Irritability Leaf-mosaic Drip-tips. 



WE may now consider the life-actions i.e. the physiology 

 of the leaf as a whole. 



As we have seen, the leaf is in its typical form an 

 organ adapted by its position, extension, shape, structure 

 and other peculiarities, for the suitable exposure of the 

 cells containing chlorophyll-corpuscles to the access of 

 water, containing traces of minerals, and of air, containing 

 traces of carbon-dioxide, and also of the light from the 

 sun which is essential for the purpose of enabling the 

 machinery of the chlorophyll-corpuscles to do its work 

 of carbon-assimilation the photo-synthesis which results 

 in the building up of the organic substance of the plant. 

 All our interest is primarily centred around this principal 

 function of the leaf; and although many other functions, 

 subsidiary to this one, are performed by the leaf in order 

 to enable it to attain the object for which it is specially 

 adapted, these secondary or subordinate functions ought 



