70 TISSUES 



class of feature selected in a given case for purposes of definition and 

 classification. 



The physiological anatomist must of logical necessity define and 

 arrange the various tissues of the plant-body in accordance with their 

 anatomico-physiological characters. In so doing he starts with the 

 assumption that the differentiation of a variety of tissues within the 

 plant-body is mainly the outcome of division of labour, and that 

 consequently the most characteristic features of each tissue are those 

 which are most intimately connected with its physiological activity. 

 The recognition of a physiological function does, in fact, presuppose 

 the existence of a definite anatomical structure which is adapted for 

 the performance of that function. Hence, every morphological feature 

 of a tissue which can be shown to be thus adapted may be regarded 

 as an anatomico-physiological character. Evidently, therefore, the 

 anatomico-physiological definition and arrangement of tissues provides 

 the broadest and most natural of all systems of tissue-classification, since 

 from this point of view the plant-body is regarded not merely as a more 

 or less complex aggregate of formal elements, but also as a living- 

 organism, composed of a number of functional units and engaged in a 

 corresponding number of physiological activities, which all contribute 

 to the safety and welfare of the whole. 30 It does not follow, however, 

 that no other classification of tissues and tissue-systems is admissible 

 in any circumstances. All that is required of a scheme of classification 

 is that it should be carried out consistently from a single point of view. 

 Those arrangements, on the other hand, which are merely intended to pro- 

 vide a simple and convenient list of the various forms of tissues are devoid 

 (if scientific value, however useful they may be for didactic purposes. 



In defining and classifying the various tissues of the plant-body 

 the physiological anatomist takes no account of their ontogenetic or 

 phylogenetic relations. The homologies of tissues are of no interest 

 to him in this connection ; his concern is solely with analogy. A single 

 anatomico-physiological system such as the mechanical system may, 

 therefore, comprise tissues of very diverse ontogenetic and phylogenetic 

 origin ; it nevertheless represents a homogeneous structure from the 

 present point of view, inasmuch as the various tissues included in the 

 system are all concerned with similar functions. Conversely structures 

 which have a common ontogenetic or phylogenetic origin may be 

 assigned to systems that are widely separated in the anatomico-physio- 

 logical scheme. Thus epidermal fibrous strands such as are found in 

 certain Cyperaceae, stomata, epidermal hydathodes, root-hairs, etc., all 

 pertain to the epidermis in the ontogenetic sense, whereas in the present 

 work they are properly relegated to totally distinct tissue-systems. 



Before proceeding to apply the principles outlined above to a pre- 



