256 PROTOPLASM 



cellulose. The same striated and articulate structure of cellulose 

 persists in bituminous coal, as shown in Spierer photographs 

 by Thiessen (Fig. 1365). The fibers in wood are also built 

 up of the fibrillae characteristic of plant cellulose in general. 



When beaten wood (paper) pulp is examined under the 

 microscope, it is seen to consist of many parallel fibrillae which 

 can be dissected out with the aid of microneedles. 



Natural cellulose thus consists of units of ever increasing 

 size, all of which, from chain molecules to visible wood fibers, 

 are of linear form. The orientation of these units determines 

 the physical properties (elasticity, tensile strength, etc.) of the 



Fig. 136. — A, the striated structure of the cellulose wall of plant cells as 

 revealed by the Spierer lens; B, similar structure of bituminous coal seen with 

 the Spierer lens. (From H. R. Thiessen.) 



material. Where the orientation is orderly and parallel to the 

 fiber axis, the tensile strength is high. Lack of symmetry in 

 arrangement means low tensile strength. The force responsible 

 for orientation may be the purely mechanical one of strain, 

 as in rubber, which unstretched shows no X ray diffraction 

 pattern but stretched becomes crystalline; or the orientation may 

 be due to polarity, a very prevalent property of matter. 



Polarity. — On a number of occasions we have seen that polarity 

 is a property involved in many reactions, for example, it deter- 

 mines the orientation of (polar) molecules at surfaces (page 130). 

 Probably no other force in nature is so widely distributed and 

 plays so great a role in the behavior of systems, from molecules 

 to organisms, as polarity. The term expresses any situation 

 where the two ends or sides of an object are different. A wil- 

 low cutting shows its polarity when it grows roots at one end 

 and leaves at the other. A molecule is polar if its ends differ. 

 Strictly, polarity should be limited to objects the ends of which 



