2 



PRINCIPLES OF PLANT-TERATOLOGY. 



the bases of the leaves.* Further, these three cate- 

 gories have always, in spite of many apparent inter- 

 mixtures and transitions, remained distinct from each 

 other. The diversity of form — the well-nigh endless 

 modifications which these three organs have under- 

 gone owing to their equally varied adaptations and 

 uses — has misled botanists into imagining either that 

 there are more than three categories of organs or that 

 no hard and fast line exists between any two of them, 

 so that stems may change into roots, roots into stems, 

 leaves into stems, etc. But no one has ever yet 

 demonstrated the existence of any such transitional 

 forms, i. e. forms which, at one stage of their indi- 

 vidual existence, can be shown to be of the nature of 

 one of the organ-categories, and at another stage can 

 be shown to partake of the characters of another 

 category, with intermediate steps between the two. 



Goebel says : " Formerly much controversy turned 

 on the question as to whether a sporangium or an 

 ovule should be considered as a modified hair (trichome), 

 or whether it should rank as a foliar (phyllome), or 

 axial (caulome) structure. At the present it is 

 regarded as immaterial whether it corresponds in 

 position with any one of these organs ; its essential 

 importance is discerned in its spore-producing char- 

 acter, that is, ultimately, its functional property." 

 But, while the function of this sporangium and ovule 

 is of great importance, it is surely of not less import- 

 ance to trace if possible the evolutionary history and 

 origin of these organs and to discover their homo- 

 logies with other organs. 



The Methods of Morphological Investigation. — 

 In order to determine the origin, from one of the 

 three categories of organs, of any structure whose 

 present nature is doubtful, and whose mere appear- 

 ance is perhaps wholly misleading, there are four 

 methods of investigation from which we may choose. 



In the first place there is the study of the ontogeny 



* This is the phyton-theory, which is here upheld. 



