82 IMBIBITION AND MOLECULAR STRUCTURE 



Optical properties, as far as they indicate differences of molecular structure, may 

 attain considerable importance in physics and chemistry '. A few remarks must, 

 however, here suffice, a knowledge of the phenomena and causes of double refrac- 

 tion being assumed on the part of the reader, while for further details he is referred 

 to the literature on this subject 2 . 



The protoplast, saturated with water, is isotropous, but several of its products, 

 cell-walls, starch-grains, and crystalloids, are anisotropous, and as dead substances 

 retain the same optical properties. 



When first formed the cell-wall is either not at all, or only very slightly doubly 

 refractive. Marked anisotropy only appears as it becomes older, and is not always 

 due to precisely the same causes. In the cuticle, as Ambronn 5 has shown, the 

 anisotropy is caused by an infiltration of doubly refractive waxy substances. When 

 warmed, these melt, and the anisotropy disappears ; when cooled, they congeal, and 

 the anisotropy returns. In general, the optical effects are probably due to the aniso- 

 tropy of the micellae or micellar complexes. Nevertheless, it is always possible 

 that in some cases double refraction is partly or entirely due to differences of 

 tension existing between the successive layers. 



Since the smallest fragments of a cell-wall or starch-grain still show double 

 refraction, their anisotropy cannot possibly be due to differences of tension existing 

 only in the intact wall or grain. In many cases the optical effect is hardly, or 

 not at all, altered by applying marked tension or strain, but in other cases, as 

 shown by recent research, such treatment causes a marked alteration of the optical 

 properties 4 . Swelling may also produce either an imperceptible or a very marked 

 optical result. Usually as the swelling increases the double refraction decreases 5 ; 

 indeed some bodies, such as softened gelatine, become doubly refractive only when 

 dried. 



In cases where double refraction is due to the component elements or micellae, 

 these must be arranged for the most part, or entirely, in a definite manner. Thus 

 in a cylindrical cell one axis of the optical ellipsoid of elasticity is radial, the other 

 two tangential to the surface, and at the same time they form acute or right angles 

 with the main axis of the cell. The shortest optical axis falls in the plane in which 

 the greatest swelling is possible *. From this fact, and from the way in which the 

 arrangement of the optical axes corresponds with the direction of the visible mark- 

 ings, it is justifiable to conclude that all these appearances are produced by the 

 same specific internal structure. 



1 Cf. Ostwald, Lehrb. d. allgem. Chemie, 1891, 2. Aufl., Bd. I, p. 460. 



" Ambronn, Einleitung z. Benutzung d. Polarisationsmikroskops, 1892 ; Nageli 'und Schwen- 

 dener, Mikroskop, 1877, 2. Aufl., p. 299; Schwendener, Sitzungsb. d. Berl. Akad., 1877, p. 659; 

 1889, P- 2 33 i l $9, ? US* J Ebner, Unters. ii. Ursachen d. Anisotropie, 1882, and Sitzungsb. d. 

 Wien. Akad., 1885, Bd. xci, Abth. 2, p. 35 ; Correns, in Beitrage z. Morphol., &c., v. Zimmer- 

 inann, 1893, Bd. I, p. 302. The works by Nageli are in Bot. Mitth., Bd. I, 1862, p. 183, &c., and 

 Beitrage z. wiss. Bot., 1863, Heft 3, p. i. In the above works the literature is quoted, including that 

 which seeks to refer all double refraction to unequal tension. 



3 Ambronn, Ber. d. Bot. Ges., 1888, p. 226. 



* Zimmermann, ibid., 1884, p. 35 ; Schwendener, 1. c., 1887, p. 687, and 1889 ; Ebner, 1. c. 

 5 Schwendener, 1. c, 1887, p. 695. 



* Nageli und Schwendener, Mikroskop, 1877, 2. Aufl., p. 356, and the literature cited above. 



