14 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



theless but few cases where any cloubt exists as to the category to which 

 a definite organ belongs, in other words, its morphological significance or 

 ' homology ' is clear. This notion of homology I must now discuss. 



Just as in systematic botany no single character can be considered as 

 a critical mark of affinity, so also no single mark can be taken as a 

 criterion of the homology of an organ, but it is groups of peculiarities 

 which are of morphological value and give us the key to homology. 

 What has to be determined is the position of an organ in the whole 

 development, to what organ of an allied form it corresponds, through 

 what transformations it has passed, that is to say, what change of function 

 has befallen it. We shall see hereafter to what unprofitable speculations 

 the neglect of these principles has led, especially in the consideration of 

 the organs of propagation. It is these organs which occasion one of the 

 greatest of the difficulties which arise in the way of the definition of 

 the vegetative organs. The special organs of propagation sporangia, 

 oogonia, and the like exhibit in the nature of their case no change in 

 their function ; their function and form are fixed, and this is what gives 

 them their chief value in systematic work. 



It will be useful if I refer here to the endeavours which have been 

 made to fix the limitations of organs of plants, because the question itself 

 is of importance and much uncertainty even now prevails regarding it '. 



1. It is manifest that the distinction of organs must have originally 

 been based upon differences of outer form. The word ' blade ' indicates 

 that the original conception of a leaf was that of a flat organ which was 

 distinguished by this character from the usually cylindric stem ; under 

 the designation root, all subterranean organs were reckoned. It is 

 however now generally known that there are leaves which have all the 

 appearance of shoots, and the converse is also the case. Nevertheless, in 

 one of the most recent text-books the leaves of the rush are designated 



* o 



leafless ' shoot-axes ' because they have the appearance of cylindric leafless 

 shoots, and in the same work the rhizoids of the moss are termed ' hairs ' 

 only because they have the appearance of the hairs of higher plants, 

 and although they have otherwise nothing whatever to do with them. 



2. External form is closely connected with function and with ana- 

 tomical structure. In the vegetative organs the form may change, accom- 

 panied by a change in anatomical structure ; ' metamorphosis ' may take 

 place, and a flower-leaf is the homologue of a foliage-leaf notwithstand- 

 ing that it has quite a different form. The anatomical structure of 

 homologous organs is often very different, and the attempts which have 

 been made to prove upon anatomical grounds the leaf-like twigs of 



1 Consult Goebel, Vergleichende Entwicklungsgeschichte, p. 127; Bower, On the limits of the use 

 of the terms ' Phyllome ' nml ' Caulome,' in Annals of Botany, i. p. 133. 



