188 PHYSIOLOGICAL CLASSIFICATION OB^ TISSUES. 



MECHANICS OF TISSUES. 



520. In Haberlandt's classification 1 the tissues having a me- 

 chanical office to fill are brought into one group, which is then 

 subdivided into (1) those tissues which protect the softer tissues 

 of the interior from the harm which would result from exposure, 

 and (2) those which hold the soft tissues in place. An exami- 

 nation of the work performed by tissues ma^y accompany an in- 

 vestigation of the work by organs themselves ; in the examina- 

 tion of the work of organs in Part II. the necessarv facts relative 



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to their structure will be presented. 



521. Those tissues which serve simply to impart strength to 

 the plant belong almost as much to lifeless as to living parts, and 

 can best be examined before the subjects of physiology are taken 

 up. The present division has for its object the consideration of 

 that which in Haberlandt's classification is called the skeleton, 

 and which is known to serve chiefly mechanical ends. 



522. In the case of a water-plant, for instance an alga, which 

 has about the same specific gravity as the water in which it is 

 borne, no special mechanical support is demanded. Its own 

 buoyancy suffices to keep the structure as a whole in place ; 

 while the different parts of the simple organism have a degree of 

 stabilitv which enables them to resist the action of the waves. 



V 



As might be expected, such an organism can attain a very great 

 size ; for instance, Macrocystis pyrifera of the Southern Pacific 

 Ocean has been known to measure nearly one thousand feet, and 

 less trustworthy measurements have been recorded which far 

 exceed this. In this and other water-plants the medium which 

 buoys the plant up takes the place practically of any internal 

 framework. 



523. A land-plant, existing in a far lighter medium than the 

 water-plant, must have a definite mechanical support. Those 

 species of Calamus which furnish the " rattan " of commerce pos- 

 sess a terminal shoot from which are unfolded in rapid succession 

 strong: leaves armed with recurved hooks. Having reached the 



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thickly clustering tops of a tropical forest, the terminal bud de- 

 velops its leaves, and these cling with tenacity to the branches 

 upon which they rest, so that the mechanical support is afforded 

 in this case b}~ the vegetation beneath. Thus supported, the ex- 

 tension of the shoot is indefinite, so that examples of Calamus 



Physiologische Pflanzenanatomie (Leipzig, 1884). 



