924 A TEXTBOOK OF THEORETICAL BOTANY 



completely resistent to changes in the environment, and there is little in the 

 vegetative kingdom which accords with the doctrine expressed by the German 

 word " anlage," which involves the idea of a predetermined development, 

 supposed to be inherent in a given organ rudiment. 



We shall select certain outstanding examples of habitual modifications, 

 without regard to the question of their fixity or mutability. The larger works 

 of systematic Botany will provide the student with countless other instances. 



Modifications of the Aerial Shoot 



J . Cladodes and Phylloclades. The term cladode is applied to a branch 

 of a single internode which is flattened to simulate a leaf, and the term 

 phylloclade to entire shoots similarly flattened and leaf-like. There are 

 considerably more cases in the latter category, and it is not always easy to 

 discriminate the one type from the other, with the result that many botanists 

 treat them as synonymous. 



The biological principle is the same in both, namely the replacement of 

 reduced leaves, which are no longer functional for photosynthesis, by flat 

 branches, containing chlorophyllous tissue, which function physiologically 

 as leaves. The benefit to the plant of such a procedure is indeed puzzling, 

 inasmuch as the cladodes are never as efficient photosynthetically as are true 

 leaves. Perhaps we have here an example of the supposed " I^aw of Irre- 

 versibility " in evolution, which maintains that a structure once lost cannot 

 be regained but can only be substituted by the modification of other existing 

 structures. As such a substitution necessarily involves the production of 

 new characters in the structures concerned, it is difficult to understand what 

 should prevent the redevelopment of the lost character as the simplest response 

 to the demand of necessity. But to these questions there are no adequate 

 answers. 



A good example of a simple cladode is that of Ruscus aculeatus (Butcher's 

 Broom) (Fig. 907) and of some related plants such as Semele and Danae. 

 The cladodes are short, ovate, spiny-pointed and with parallel veins. They 

 are deep green in colour and arise in the axils of minute scale leaves, which 

 soon drop off. In the young shoot of Rusais these leaves are much bigger 

 and partly green ; the cladode habit is only fully established in mature shoots. 

 Each cladode bears, half-way up and in the median line, a scale-like bract, 

 from the axil of which arises in summer a group of flowers. 



There has been much dispute about the morphological nature of this 

 structure. The classical view is that the whole structure is a branch of 

 limited growth bearing one node from which arise the flowers, subtended by 

 the small bract. Opposed to this is the view that only the lower half is axial 

 in nature and that at its apex (the flower bearing " node ") are two opposed 

 bracts, one small and one large, the latter forming the upper half of the 

 cladode, with its margins decurrent along the lower, axial portion. This 

 latter view, though it seems far-fetched is, in fact, supported by the vascular 

 structure, which is axial only in the lower and leaf-like in the upper half, and 



