852 BOTANY AND PHARMACOGNOSY. 



Habit : Crystals commonly tabular on the base and appear- 

 ing as square plates. Occasional crystals are seen in the side 

 aspect showing the pyramid (221). This face does not appear 

 as a smooth plane, but as a rough striated surface more or 

 less curved, the striations being parallel to the base (Fig. 343). 



Optical properties : Uniaxial, optical sign . Small crystals 

 in the usual aspect (on the base) show no polarization effect. 

 Larger crystals show a faint illumination between crossed nicols, 



Fig. 344. Strychnine sulphate: tetragonal crystals from aqueous solution, showing 

 basal aspect in polarized light. Also one large crystal of a second form (orthorhombic?) 

 of strychnine sulphate. 



owing to optical anomalies and to the rotation of the plane of 

 polarized light, which amounts to about 10 per mm. thickness. 

 Crystals seen on the side view show a strong double refraction. 



Vanillin (CgHgOa). 



Occurrence: See pp. 587, 672, 673 and 680. See also 

 Czapek.* 



* Czapek, Biochemie der Pflanzen, ii, p. 551. 



