CHAPTER IV. 



THE LEAF. 



Everyone knows roughly what is meant by the term Leaf. It is 

 commonly a fiat, stalked structure, which exposes a large area of 

 green tissue to the hght and air, usually in a more or less horizontal 

 plane ; and in a great many of our native plants it falls off in the 

 autumn by detachment at the base of the leaf-stalk. But this general 

 conception of the leaf is not of universal application, and it does not 

 define the leaf in a scientific sense. The essential features of the leaf 

 are not found in its form or direction, or in its detachment in autumn, 

 for all of these are liable to vary. The features that are constant for 

 leaves^ and so define them morphologically, are that they arise as 

 lateral outgrowths from the apical cone, that they spring from the super- 

 ficial and underlying tissues, and arise in acropetal succession ; also 

 that they do not repeat the characters of the shoot itself. It follows 

 from their origin that their tissues are continuous at the base with 

 the tissues of the axis. The conceptions of axis and leaf are cor- 

 relative : the axis is a spindle bearing leaves, and the leaf is a 

 lateral appendage upon the axis. Axis and leaves together constitute 

 the Shoot. This conclusion holds notwithstanding that both stem 

 and leaf may vary greatly in form and proportion. Particularly the 

 leaf is variable in outhne : it may have a stalk or petiole, which bears 

 the blade or lamina at its end : or there may be no stalk, and the blade 

 is then seated directly upon the axis. Further, the blade itself shows 

 the greatest variety in outline ; it may be a simple flat expansion 

 (entire), or it may be cut up in various ways into parts (pinnae), and 

 these may be again subdivided. We need not follow this further than 

 to recognise the fact that the outhne of the leaf varies greatly. It 

 may even show differences in the individual, as is seen on comparing 

 the bud-scales and fohage leaves of any ordinary plant with those 



58 



