MECHANICAL AND CONDUCTING STRANDS 169 



the organ which they serve to strengthen almost invariably reveals 

 the dominating influence of the mechanical principle. Incidentally, of 

 course, the mechanical strands also display relations to other anato- 

 mico-physiological systems, and these relations must be taken into 

 account, before one can comprehend every detail in the structure and 

 arrangement of the mechanical system. These secondary relations 

 will he referred to again later on. At the present stage it must 

 suffice to diaw attention to the frequent association of fibrous strands 

 with vascular bundles, a phenomenon the true significance of which 

 was long misunderstood, it is very usual, namely, for a fibrous 

 strand to apply itself closely to a vascular bundle, the resulting com- 

 plex morphological entity being termed a fihro-vascular bundle. In 

 this case the fibrous strands, when seen in transverse section, appear 

 as semi-lunar or sickle-shaped borders or partial sheaths embracing 

 one or both halves of the vascular strands, or as complete sheaths 

 which entirely surround the bundles. Schwendener was the first to 

 point out that the association of bast with vascular tissue in these 

 fihro-vascular bundles is not the effect of some obscure morphological 

 law, hut really represents a case of physiological opportunism. 



Since a vascular bundle is in part composed of very delicate cells, 

 it requires to he protected by some coarser and more resistant tissue, 

 and hence tends to associate itself with mechanical strands. A 

 gutter-shaped fibrous sheath acts like a splint or greave, behind which 

 the delicate parts of the vascular bundle find the necessary shelter. 

 Vascular bundles are, on the other hand, particularly well fitted to 

 form the web between two flanges of hast, more especially in the case 

 of small girders, so that the association of fibrous and vascular tissues 

 appears advantageous from this point of view as well. The term 

 mestome signifying "filling tissue" was applied by Schwendener to 

 unmixed "vascular" bundles, that is, to conducting strands which are 

 entirely free from any admixture of mechanical elements, " in order 

 to give expression to the fact that vascular bundles are so frequently 

 interpolated between the flanges of girders, or else embedded in a 

 complete cylinder of hast, or finally scattered throughout a continuous 

 mass of wood-fibres, as in woody Dicotyledonous stems, where the 

 conducting strands seem to occupy the gaps ami interstices of a 

 mechanical framework." In the present work it will he regularly used 

 in referring to these relations between the vascular and mechanical 

 systems. 



A. THE STRUCTURE OF INFLEXIBLE ORGANS. 



Most aerial plant organs are constantly exposed to bending stresses. 

 In cylindrical structures such as Grass-haulms, floral axes, etc., the 



