NOTE ON CRYSTALS 103 



be seen to close, the guard-cells straightening themselves as 

 their internal tension is relieved, and finally becoming plasmo- 

 lysed. The connection between the opening of the stoma and 

 the internal tension of the guard-cells is thus demonstrated. 



Note on crystals. 



I. Cut longitudinal sections of the scape or leaves of the 

 Hyacinth or Onion : many other Monocotyledons will do as 

 well : mount in water, and observe the large cells containing 

 numerous needle-shaped crystals (Raphides) arranged in a 

 bundle parallel to one another. In order to investigate their 

 nature the following tests may be applied : 



a. The attempt may be made to stain them with iodine, 

 or other stains which colour crystalloids, but they will 

 not be affected : they are thus distinguished at once from 

 crystalloids. 



b. Irrigate a section with acetic acid : they are not affected : 

 they are therefore not calcium carbonate. 



c. Irrigate with dilute nitric acid : the crystals are dissolved. 



d. Irrigate a fresh preparation with a small quantity of dilute 

 sulphuric acid : the crystals will be dissolved, and crystals of a 

 different form (calcium sulphate, which is not readily soluble) 

 may be seen to be formed in the fluid. 



These reactions, coupled with what can be ascertained from 

 analysis of the ash of the plant, point to the conclusion that 

 these crystals consist of calcium oxalate. 



II. Cut transverse sections of the petiole of some species of 

 Begonia; mount in water, and examine under a low power. 

 Here and there will be found bodies of a more or less distinctly 

 crystalline form occupying the cavities of certain cells. Their 

 form is very complicated, and their size variable. 



The reagents above applied to the Hyacinth are to be used : 

 the results will be similar. Thus they also may be shown to 

 consist of calcium oxalate. Crystals giving the above reac- 

 tions will be found in the tissues of most plants. 



