INTRODUCTION 



of the environment, as may happen, for example, in an equably moist 

 and warm tropical climate. 



Even in its mildest form, however, the struggle for existence is 

 active enough to prevent the perpetuation of a large proportion of 

 useless morphological peculiarities in addition to the stock of useful 

 adaptations with which every organism is provided. In fact it would 

 not he going too far, altogether to deny the existence, in the internal 

 organisation of plants, of structural units whether single cells or entire 

 tissues -which have always been functionless. On the other hand, 

 the diversity which evidently characterises the detailed construction of 

 organs of ever} type, is doubtless often nothing but the expression of 

 an inherent "' creative tendency." We are thus brought face to face 

 with the problem of alternative designs (Konstruktionsvariationen), a 

 phenomenon which is of widespread occurrence in the vegetable king- 

 dom. An engineer may construct several bridges of the same span, of 

 equal strength and at a similar cost, and follow a distinct plan in each 

 case. Similarly, in the organisation of a plant, one and the same 

 tissue-system may lie capable of construction according to several 

 distinct plans, none of which offers an appreciable advantage in com- 

 parison with the rest. Nature, as it were, takes a pleasure in ringing 

 the changes indefinitely upon the possible variations of structural 

 detail; in this way arises the inconceivable diversity which prevails 

 with regard to the details of anatomical structure. Not infrequently, 

 indeed, a definite advantage does result from the choice of one among 

 several possible methods of construction. Thus the isolation of the 

 subepidermal fibrous strands which is characteristic of the haulms of 

 the genus Juncii* (and also of many Cyperaceae) provides ample room 

 for the intercalation of strips of photosynthetic tissue, and renders 

 it easy for the haulm to assume the role of a photosynthetic organ. In 

 many cases, however, it is a matter of indifference as far as the welfare 

 of the plant is concerned, whether one plan of construction is selected 

 rather than another ; it would, therefore, be futile to attempt an explana- 

 tion of every case of alternative design on strictly utilitarian lines. 

 The palisade-structure characteristic of specialised photosynthetic tissue, 

 For instance, may be produced in two quite distinct ways, namely, 

 either by the development of inwardly directed folds of the cell 

 membrane, or else by the formation of complete partitions, each con- 

 sisting of an entire cell wall. The two types of structure may be 

 mutually exclusive as regards their occurrence within a group of closely 

 related plants {e.g. in the family RANUNCULACEAE), or both may be found 

 in the same individual; there is. therefore, not the slightest reason to 

 suppose that the two varieties of structure represent separate special 

 adaptations. What the plant really requires in either case is the 



