INTRODUCTION. MORPHOLOGY AND ORGANOGRAPHY 



which has not developed further, but has died off. This interesting fact 

 is explained when we follow the history of development of the foliage-leaf, 

 which need only be looked at here in its coarsest features. At a certain 

 stage of development of the foliage-leaf its form is something like what 

 is shown at Fig. i, IV. We recognize clearly the rudiment of the 

 leaf-blade, L, and its lobes are already visible ; the leaf-stalk has not yet 

 appeared, but it will arise by the elongation of the zone lying between 

 the leaf-blade, Z, and the leaf-base, 6". This stage of development is 

 found in both foliage-leaf and 

 bud-scale. But after this a 

 change sets in : the primor- 

 dium of the blade, L, dries up, 

 the leaf-stalk is not developed, 

 whilst the leaf-base grows 

 considerably and itself forms 

 the scale-leaf from which we 

 started. 



We are able, as I have 

 already stated, to control 

 experimentally the develop- 

 ment of these leaf-primordia ; 

 one that would on account 

 of its position develop nor- 

 mally as a' scale-leaf we can 

 force to become a foliage- 

 leaf, and the converse is also 

 true. It happens also, and 

 this will be shown in the 

 special part of this book, that 

 the primordium of the blade 

 may become arrested at a 

 late stage of development. In 

 Brownea erecta, for example, 

 the buds are protected by 

 erect dried-up foliage-leaves, 

 the pinnules and other parts 



of which are clearly formed. It may frequently happen that the trans- 

 formation takes place much earlier and is then of course no longer capable 

 of direct proof, but only to be concluded upon comparative grounds. 

 When now the differentiation theory says the primordium of a scale-leaf 

 and of a foliage-leaf of Acer are both ' leaf-primordia ' which may 

 sometimes develop in this way and sometimes in that, it entirely over- 

 looks the point that the notion of ' leaf is a purely abstract one, that 



FIG. i. Acer platanoides. /Foliage-leaf reduced ; deaf-base, 

 .$ leaf-stalk. // Bud-scale. /// Young bud-scale, magnified ; 

 L primordium of the leaf-blade which is arrested later. IV 

 Primordium of foliage-leaf, enlarged and diagrammatic. 



