GENERAL PRINCIPLES OF CONSTRUCT] ON 65 



for the anatomical and topographical features are always referable to a 

 common determining influence, to wit, the nature of the functions 

 performed by the tissue. Although the detailed consideration of the 

 principles embodied in the structure and arrangement of the several 

 tissues and tissue-systems forms the subject-matter of the succeeding 

 chapters of the present work, it is nevertheless desirable to devote 

 a few words at this stage to four outstanding general principles which 

 control the entire architectural plan of the plant-body, or which, 

 at any rate, determine the leading features of a number of tissue- 

 systems. 



Reference must first of all be made once more to the principle of 

 division of labour, which has already been mentioned on more than one 

 occasion. It is mainly through the consistent application of this 

 principle that plants and animals are enabled to perform their mani- 

 fold functions in an efficient and reliable manner. Where every 

 important function is assigned to a specialised organ or tissue, the 

 morphological structure of each separate apparatus, and the function 

 allocated thereto, can be mutually adjusted with the highest degree of 

 precision. Physiological division of labour thus naturally leads to the 

 morphological differentiation exhibited to a greater or less extent by 

 the organs and tissues of every plant. The plants which stand at 

 the highest level of organisation are those in which this principle 

 of division of labour is carried out in the most thoroughgoing and 

 consistent manner. 



A second factor of fundamental importance, the influence of which 

 is clearly perceptible in the structure of almost all the organs and 

 tissues of the more highly organised plants, is the so-called mechanical 

 principle. It is obvious that a certain degree of mechanical 

 strength constitutes not only an essential feature of the general 

 architectural plan of every plant, but also an indispensable condition 

 for the undisturbed normal activity of each individual tissue and 

 organ : similarly, in the case of a house, not only the arrangement 

 of the foundations and roof, but also every detail of construction 

 must satisfy a variety of mechanical conditions. Consequently, every 

 highly-organised plant, besides developing a principal mechanical 

 system or " skeleton," possesses in addition a number of minor 

 mechanical arrangements, the majority of which are of purely local 

 importance. Many of the histological details of the dermal, the 

 photosynthetic and the conducting tissues, in particular, reveal the 

 dominating influence of this mechanical principle in the clearest 

 possible manner. 



The principle of economy of material is most important from the 

 ecological point of view. Owing to the intensity of the struggle 



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