CHAPTER XIII. 



LEAF-CASTING AND THE FORMATION OF 

 LEAF-SCARS. 



Relations of leaf to shoot-axis Venation Fibres and vessels 

 Continuity of the vascular system Fibro- vascular skeleton 

 Comparison to girders, water-supply arid drainage systems 

 Fall of leaves Separation-layer or absciss-layer Leaf-scar. 



THE study of the leaf as a physiological organ of the 

 plant will be deferred for another section of this work, but 

 it is necessary at this stage to emphasize some aspects of 

 the relations between the shoot-axis and the leaves borne 

 on it, in order the better to understand certain marks on 

 the twigs in their winter condition. 



Our examination of the bud has shown us that the 

 leaves are outgrowths of the axis, and that as the axis 

 elongates the leaves also grow longer and broader. When 

 the leaf is completed we notice that it is in typical, that 

 is to say ordinary cases, a flattened sheet of green tissue 

 with harder parts running in its substance in the form of 

 much branched networks, termed the venation of the leaf 

 (Figs. 19, 26, 48 and 49). 



This venation consists essentially of numerous very 

 thin pipes or tubes employed for conducting fluids from 

 one part of the leaf to another, and called vessels ; and of 



