914 



A TEXTBOOK OF THEORETICAL BOTANY 



damage on the neighbouring tissues. A comparison of several plant fibres 

 with metal wires is given below, from Schwendener's figures : — 



Schwendener interpreted the distribution of mechanical tissue, by a 

 comparison with the ironwork of his own day, as representing an arrangement 

 of girders. 



In the ordinary I-girder the top and bottom portions are called the 

 " fianges," and the upright middle portion the " web." When such a girder 

 is subjected to a bending strain in the plane of the web it is the two fianges 

 which supply the greatest part of the resistance ; the web is of minor 

 importance and may be made of much lighter material. The flange nearest 

 the side of the applied force is subject to an extension strain, while the other 

 suffers a compression strain. Midway between them is the null-line, where 

 neither strain operates and which therefore provides no part of the resistance. 

 The strain in each portion of the cross-section of the girder is proportional 

 to its distance from the null-line. 



The same principles apply to the resistance of a cylindrical column of 

 homogeneous material, and it is obvious that the strength of the column 

 must depend principally on the strength of its outer zones. The centre may 

 indeed be hollow, i.e., tubular, with very little loss of strength. 



Schwendener analyzed the mechanical tissues of stems and leaves accord- 

 ing to these principles. He classified the structures found in Monocotyledons 

 alone into twenty-eight mechanical types, each with a characteristic girder 

 structure. His girders are nearly all made up of sclerenchymatous fianges, 

 with xylem or collenchyma tissues forming the webs. In some cases the 

 girders are isolated fibro-vascular bundles, in others they are united side by 

 side into tubes, sometimes supported by radial fianges, while in some cases 

 either separate fianges or whole girders form opposing systems across the 

 stem. A good example of such girder construction is shown in Fig. 896. 



Engineering practice has advanced since 1874, and in the light of experi- 

 ence in the design of ferro-concrete structures, it is possible to get a fuller 

 idea of the mechanical principles of plant construction. The weakness of 

 Schwendener's theory was that it treated the sclerenchyma as if it was an 

 isolated system and left out of account the softer tissues in which it is 



