2 INTRODUCTION 



There is no doubt that the rule in question holds good in the vast 

 majority of cases. At the same time the adult organs of many plants 

 certainly contain isolated cells, or even groups of cells, which must be 

 regarded as functionless in the sense that they play no useful part in 

 the general economy of the plant. Again, it may happen in the case 

 of a particular cell-group or tissue that an individual morphological 

 character cannot be " explained " with reference to a definite physio- 

 logical function, or at any rate that the two cannot be brought into 

 direct relation with one another. Such functionless units and unex- 

 plained features of the internal structure of plants are of various kinds ; 

 their discrimination is a matter of considerable importance to the physio- 

 logical anatomist, mainly because he is unable otherwise to determine 

 the natural limits of his proper field of work. 



The functionless condition may be due to loss of function. It is 

 further necessary to distinguish between ontogenetic loss of function, 

 which takes place during the development of the individual, and 

 phylogenetic loss of function, which arises in the course of evolution of 

 a species, a genus, or a family. A few examples will render the dis- 

 tinction more evident. When cork is formed, the primary dermal tissue 

 or epidermis is ruptured and degenerates ; again, in many plants the 

 stomata of the older leaves lose their power of adjustment and hence 

 cease to regulate gaseous interchange ; lastly, in the older portions of 

 secondary wood the water-conducting vessels become functionless by 

 occlusion. All these instances illustrate loss of function of the onto- 

 genetic type. 



Less numerous are the cases in which a loss of function, initiated at 

 some previous stage in the evolution of the species under consideration, 

 has resulted in the phylogenetic reduction of a given organ or tissue. 

 On becoming functionless, namely, an organ or a tissue inevitably 

 undergoes from causes which cannot conveniently be discussed here 

 a gradual reduction, which may end in complete atrophy. An illustra- 

 tion is provided by the more or less pronounced reduction of the 

 parenchymatous bundle-sheaths which prevails in the leaves of certain 

 species of Euphorbia. When typically developed, such bundle-sheaths 

 are responsible for the removal of synthetic products from the leaf. 

 In the Euphorbias referred to, this work is in part performed by the 

 laticiferous system ; as a result, the bundle-sheaths are to a large extent 

 relieved of their duties and accordingly undergo very obvious and 

 extensive reduction. 



Another instructive illustration of phylogenetic reduction is fur- 

 nished by the functionless stomata which occur on the sporogonia of 

 Sphagnum : here guard-cells are still initiated, but the pore and the 

 internal air-chamber both remain undeveloped. In this case the reduc- 



