170 MECHANICAL SYSTEM 



bending forces may act in any plane at right angles to the longitudinal 

 axis; in such cases, therefore, a system of compound girders, such 

 as has been described above, is required to produce the desired radial 

 inflexibility. In these circumstances the fibrous or collenchymatous 



strands may in general he expected to arrange themselves in one 

 or more circles near the periphery. The. centrifugal tendency of the 

 mechanical system will be most pronounced, where the dominance 

 of the mechanical principle is least affected by the claims of other 

 tissue-systems. 



Flat organs, such as foliage-leaves, are chiefly exposed to bending 

 stresses in the plane which is perpendicular to their greatest surface; 

 inflexibility is therefore principally required in that plane. The 

 desired result is attained by arranging the girders in transverse 

 series. 



Schwendener has shown that the details of the mechanical 

 construction of inflexible organs vary in an extraordinary degree, 

 especially among Monocotyledons. With the object of reducing 

 the numerous varieties of arrangement to some sort of order, Schwen- 

 dener undertook a systematic classification of the various types of 

 construction, collecting them into a number of comprehensive classes 

 called mechanical systems. In the present treatise no attempt will 

 be made to deal with all the twenty-eight types of inflexible organ, 

 which Schwendener recognised and illustrated by instructive examples 

 among Monocotyledons alone. It will only be necessary to demonstrate 

 the correctness of Schwendener's attitude by selecting a few typical 

 instances for fuller consideration. Convenience will decide whether 

 any particular example is chosen from among Monocotyledons or 

 Dicotyledons: it will, further, be regarded as a, matter of indifference 

 whether the mechanical tissues selected for consideration consist of 

 genuine bast, of wood-fibres or of collenchyma. 



1. Cylindrical organs. 



We shall follow Schwendener in including among cylindrical 

 organs all structures which are liable to bend in any direction at 

 light angles to their longitudinal axis, irrespectively of the question 

 whether the)' are truly cylindrical in a geometrical sense or poly- 

 hedral in shape. 



(a) First system. Subepidermal girders. 



The two types of arrangement belonging to the first mechanical 

 system are illustrated by the petioles and infloreseence-axos of certain 

 Aroids as regards fibrous strands, and by the stems of Laiuatae and 



