DIFFERENTIATION OF ORGANS IN SPERMAPHYTA 13 



the characters of organs ; external relations alone have been taken as 

 the chief points for consideration. But the relationships of mere form are 

 by no means the permanent ones in ' the tide of phenomena.' They 

 also change. The determination of this change, that is to say, of the 

 alterations which have taken place and are believed to take place in the 

 formation of organs of a natural group, is one of the weightiest tasks of 

 organography. If we separate function from form we are at once led 

 into altogether unfruitful speculations. Further proofs of this will be 

 given in the following pages. 



II. 



DIFFERENTIATION OF ORGANS IN THE 

 SPERMAPHYTA. 



The higher plants, as the forms of vegetation which were earliest 

 studied, naturally supplied the material for the first identification and 

 terminology of the organs of plants. When the ' Doctrine of Metamor- 

 phosis ' established the conception that the many and varied organs 

 possessed by these plants were all referable to but a few ' primary forms,' 

 the definition and limitation of these became a necessity. Hence arose 

 a morphology based upon the consideration of the form of the vegetative 

 organs alone, which, while it could determine the sporophyll of the flower 

 to be simply a 'leaf-organ, could not recognize the significance of 

 the sporangium ; this was only possible by comparison with the Pterido- 

 phyta. 



The outcome of early and simple observations was the recognition 

 of root, stem, and leaf (foliage-leaf) as the chief vegetative organs of the 

 higher plants. To these organs was added subsequently the hair, a 

 structure springing from the epidermal cells and appearing as an appendage 

 of the surface. When it was found that in the construction of many 

 prickles and glands layers of the tissue deeper than the epidermis were 

 involved, the term 'emergence' was coined for them a term, the definition 

 of which is framed upon essentially negative characters : emergences are 

 neither leaves, nor shoots, nor roots, and are not endogenetic. Afterwards 

 when the stem, the leaf, and the hair, were considered as abstractions, and 

 apart from all their varying special forms, they received the designation 

 respectively of caulome, phyllome, and trichome. Although morphology 

 has never yet succeeded in framing generally applicable definitions to 

 distinguish these categories of organs from one another, there are never- 



