8 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



it is an artificially constructed category which has no concrete existence. 

 What we see is foliage-leaves, scale-leaves, tendril-leaves, sporophylls, &c. 

 That which these organs have in common, and which we endeavour 

 to fix through a general idea, must be something else than their origin 

 from ' leaf-primordia.' Seeing that if there are no leaves l there 

 can be no primordia of leaves, either the primordia of foliage-leaves, 

 scale-leaves, &c., are different from the outset, or they are alike for 

 a time and then become different, and there must therefore be an 

 actual transformation, a change in the course of development of one of 

 these primordia out of which then the others can develop. That the 

 primordia of the organs at the vegetative point are not of an indifferent 

 nature, that they do not consist merely of embryonal tissue which is 

 capable of development in any direction, is shown by the fact that the 

 primordia of the leaves and of the lateral shoots are different from one 

 another even at the moment when they appear as unmembered papillae 

 upon the surface of the vegetative point. No case is known in which a 

 papilla has developed into a shoot when its position indicated that it ought 

 to be a primordium of a leaf, and the converse is also the case. And 

 yet, as the remarkable example of Utricularia, which will be referred to 

 in detail hereafter, shows, leaf and shoot are not categories of organs 

 which are always sharply separated one from the other. We must there- 

 fore take it that primordium of shoot and primordium of leaf are usually 

 different from tJic beginning, and in this we have confirmation of the 

 conclusion already arrived at from simple analogy, that we must assume 

 for the ' primordia of leaves ' the possession from their outset of a definite, 

 not an indifferent material nature which conditions their further develop- 

 ment. This nature appears to be the same for all ' primordia of leaves ' 

 for a time 2 . Direct observation shows also that, as a matter of fact, a 

 modification of the course of development frequently occurs, that a prim- 

 ordium upon which the several parts of a foliage-leaf can be already 

 recognized may not become a foliage-leaf but something else. This 

 modification of the development always stands in relation to a change 

 of function. If the primordium of a foliage-leaf becomes a scale-leaf, it 

 has of course never discharged the function of a foliage-leaf, but it had laid 

 down the parts which are required for this function. This point may be 

 illustrated even more clearly by cases in which one and the same organ 

 shows a change of function in successive periods of its life. I will illustrate 

 this by some examples : 



The basal foliage-leaves of Lilium candidum, and the similar leaves 



1 For an instance of absolute misconception of this point and of the whole subject of transformation 

 see the remarks of, amongst others, P. Vuillemins, in L'annee biologique, i. (1895), p. 146. 



2 Compare the examples cited above. 



