CHAPTER IV. 

 THE MECHANICAL SYSTEM. 



I. GENERAL CONSIDERATIONS. 



No plant can thrive, or even maintain its existence for long, unless it 

 is provided with arrangements which ensure that the plant-body as a 

 whole is h'rnily knit together, and that each of its organs is possessed 

 of the requisite degree of mechanical strength. 



Those plants which are highly differentiated and hence made up 

 of very numerous and varied parts, are particularly subject to all 

 kinds of mechanical injury. An insufficiently strengthened organ 

 is liable, according to circumstances, to be broken across, to be torn 

 asunder, to be bruised or crushed, and so forth. Every plant must 

 safeguard itself against all the possible forms of injury by which its 

 different organs are threatened ; the latter must therefore be con- 

 structed so as to withstand, in some cases a transverse or bending 

 stress, in others a longitudinal pull, in others again, longitudinal com- 

 pressions, or radial pressures, or shearing stresses. 



Such " mechanical " arrangements are indispensable to the welfare 

 of all plants, from the most insignificant of Algae to the tallest or 

 bulkiest of trees. The thallus of every filamentous Alga requires 

 protection against the dangers of being torn asunder, of buckling and 

 of breaking across ; and it is evident that the mechanical requirements 

 which have to be satisfied by the delicate tufted Algae that grow on 

 stones in swiftly flowing mountain streams are far from negligible. To 

 take a more obvious case, a little consideration will bring to mind the 

 various demands that are likely to be made upon the strength of the 

 different organs of an ordinary forest tree. The trunk has to support 

 the weight of the massive crown, with its extensive array of branches 

 and foliage ; it must therefore be built like a pillar or column so as to 

 be capable of resisting longitudinal compression. The branches also 

 have to bear a heavy load ; these, however, on account of their oblique 



