n8 SCIENCE PROGRESS. 



of metamorphosis which, according to one definition, is 

 always associated with an alteration of function. But still 

 more evident is the conclusion to be drawn from those 

 instances where two functions gradually succeed each other. 

 A few examples will make this clear. 



The foliage leaf of Lilium candidum, and amongst 

 Dicotyledons, that of some species of Dicentra, at first 

 discharge the functions of any ordinary foliage leaf ; sub- 

 sequently its lower portion swells up to contain reserve 

 materials of food, while the more apical region dies down. 

 Here then is a real transformation of a part of the leaf 

 which alters its form together with its function. An 

 analogous case is that of a climbing plant, Quisqualis 

 ckinensis. The petioles of perfectly normal foliar organs 

 change their form, and become converted into hooks, by 

 means of which the Quisqualis shoot is enabled to climb. 1 

 In some species of Astragalus and Caragana the midribs of 

 the pinnate leaves persist, and, after the leaflets have been 

 cast off, become converted to thorns. Many other instances 

 could be quoted in which it would be impossible to deny 

 that a real metamorphosis takes place ; an organ specialised 

 in one direction assumes the function, and concomitantly 

 the form, of another. If we now suppose that the leaflets 

 of an Astragalus were to fall off before they unfolded, but 

 after their rudiments had appeared, would not this be still a 

 case of real metamorphosis ? What we term the mature 

 condition of a leaf is only the final term of a develop- 

 mental series of phenomena, and it would be erroneous 

 to conclude that the final form is the solely important 

 character of any structure ; each of the successive incidents 

 in its development is rather to be regarded as the result of 

 the preceding ones ; to designate a leaf rudiment (anlage) 

 as a mere mass of " indifferent " cells is equivalent to denying 

 that there is any cause which effects its gradual ontogenetic 

 development. It is the entire course of its progress to 

 maturity which supplies the essential characteristic features 



1 They change into hard woody structures, whilst the leaf-blade is 

 thrown off ; at first, however, they function as ordinary leaf-stalks. 



