INTRODUCTION. MORPHOLOGY AND ORGANOGRAPHY 9 



in some species of Dielytra, at first perform the functions of ordinary 

 foliage-leaves ; after a time their lower part enlarges to form a scale-like 

 tuber containing reserve-food, whilst the upper part dies off. The leaf 

 has become transformed ; it was first of all an organ of assimilation, 

 later it has become in its lower part an organ of storage. Take again 

 the case of the climber Ouisqualis chinensis. The ordinary foliage-leaves 

 perform the function normally attaching to them, but subsequently the 

 lower part of the leaf-stalk changes its form, and becomes a hard woody 

 hook, which acts as the climbing organ of the plant whilst the rest of 

 the leaf is thrown off. In some species of Astragalus and Caragana 

 the midribs of the pinnate leaves become thorns after the pinnules 

 have fallen away. *In these, as in many other cases, no one would deny 

 that an actual transformation has taken place. An organ which was 

 constructed for a definite function, and has performed this, takes on 

 another function, and acquires another form. 



Let us now assume, by way of example, that the leaf-pinnules in 

 Astragalus fall away before they unfold, and before therefore they could 

 act as assimilating organs, whilst the midrib develops into a thorn would 

 this not be a case of actual transformation ? Of course it would ; the 

 change has only been advanced a stage. What we call the mature 

 condition is only the terminal one of a series of stages of development 

 which follow one on the other. These are however not independent of one 

 another, but form a connected chain, one proceeding out of the other. 

 If we designate a primordium of a ' leaf ' at any one stage ' indifferent,' 

 that means nothing more or less than a denial of the causal connexion 

 of the developmental stages. A foliage-leaf does not become a foliage-leaf 

 only in the last stages of its development ; the material nature of the 

 primordium, whether we look for this in the existence of a definite 

 substance or of a definite structure, conditions the developmental progress. 

 This consists of phases following one upon the other, the successive ones 

 being always determined by those which precede them. Internal or 

 external influences may however divert this development into other 

 channels ; thereupon a transformation takes place. The earlier this hap- 

 pens the less is it shown in the developmental history and the more 

 different do the mature organs usually appear to be ; but in the meta- 

 morphosis of the leaf, as I have already shown, every degree of gradation 

 is found, and this furnishes the explanation of the frequent appearance 

 of formations possessing characters intermediate between two organs. 

 These intermediate stages are very common in the case of ' abnormal ' 

 transformations, which will be treated of in the Fourth Section of this book, 

 but they are also frequent as normal occurrences, in the case of bud-scales, 

 for example, as well as in other cases \vhich are easy to observe, and 

 of which I shall here mention a few. 



