TISSUES AND SIMPLE ORGANS. 67 



Let us make a simple experiment with a ruler. With one of its 

 flat sides turned upward it may readily be broken by a force acting 

 downward or upward. If the same force acting in the same direc- 

 tion acts upon the ruler with one of its edges turned upward, then 

 it will scarcely be perceptibly bent. In the latter case forces in- 

 herent in the woody fibres are brought into play to counteract or 

 equilibrate the bending force; in the former case this does not 

 occur. From this it is evident that in order to determine the point 

 of equilibrium there is besides E another magnitude, the so-called 

 moment of flexibility (Biegungsmoment). The latter ( W) is 

 dependent upon the form and area of the transverse section. In 

 the case of the ruler it is evidently the form of the cross-section, 

 which differs in the two positions. TFis found by multiplying the 

 area of each element in cross-section by the square of its distance 

 from the neutral point, and then adding the number of such prod- 

 ucts in the entire cross-section. 1 (The limit or amount of flexibil- 

 ity to be determined experimentally depends essentially upon W 

 and E.} 



From the above illustration with the ruler it follows that the 

 pressing and pulling forces (of opposite elements) resulting from a 

 lateral pressure upon a beam is inversely proportional to the distance 

 of the girdings. The supporting power of the beam increases not 

 only with the strength of the girdings, but also with their relative 

 distance of separation ; that is, the stronger the girdings the farther 

 they may be apart in order to give a maximum effect. 



This principle of the peripheral arrangement of firm elements 

 in supporting organs, though simple, is most extensively embodied 

 in multitudinous forms in the arrangement of mechanical plant- 

 cells. As it is customary in technics when using two different ma- 

 terials for example, wood and iron to place the stronger material 

 where the greatest support is to be maintained, that is, at the girding, 

 while the weaker material is used as filling, so it is found that 

 mechanical cells in the supporting tissues of plant-organs are pe- 

 ripherally arranged, while other tissues which serve the purpose of 

 nutrition, storing of food material, conduction of fluids, etc., repre- 

 sent the filling material. It is to be expected that the assimilating 

 system, dependent upon sunlight for its activity (chlorophyll-bear- 



1 Let A equal the area in cross-section of one element, r its distance from the 

 neutral point; then A. r* = the moment of flexibility of a given element. TRANS. 



