298 



FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES 



III 



redded molecules have a tendency to crystallize as thin platelets or 

 lamellae (Fig. 146a) and, when spread out, these submicroscopic, 

 often plastic, crystal lamellae are orientated. Longitudinally, they fall 

 in with the direction of the stroke and the molecules then run perpen- 

 dicular to the streak. In this wav the streaks of paraffin and beeswax 



^^ 



a) + 



b) 



c) 



W 



^ 



=-c 



/". /\ /^ 



d) 



Fig. 146. Submicroscopic textures from optically positive lipid chains. P paraflfin mole- 

 cules, E estolid molecules, W wax molecules, C cellulose chains, Cu interlinked cutin 

 chains, a) Paraffin lamella optically positive; b) optically negative streak of paraffin; 

 c) optically positive streak of pine-needle wax ; d) intercalation of cutin wax in the epidermis. 



are negative, but the molecules themselves are optically positive^. By 

 analogy it might therefore be supposed that the molecules of the 

 vegetable waxes which yield a negative streak are positive; but there 

 are some waxes with a positive streak, as I found with estolids from 

 pine needles (Fig. 146c). The streak test, therefore, tells us nothing 

 definite and another method has to be resorted to, which consists 

 in dissolving the waxes, in order that their molecules may be rendered 

 independent of each other, and then testing their intrinsic bire- 

 fringence in a flow gradient. 



Ambronn and Frey pointed out in "Polarisationsmikroskop" 

 (1926, p. 167) that the only certain way of establishing the intrinsic 

 birefringence of disperse particles is by using a rotary drum in ac- 

 cordance with Kundt's system. Signer (1930, 1933) built a flow- 



^ On p. 92 it is explained that double refraction cannot be attributed to a single 

 molecule. So if we speak here of optically positive molecules, this means that the sign 

 of the double refraction of a large number of molecules, made parallel by flow or crystal- 

 lization, is positive. 



