96 PHYSICAL CHEMISTRY 



i) Those substances which penetrate slowest are neutral salts 

 of strong acids and alkalis or alkaline earths, many neutral salts 

 of organic acids or bases, especially acids with short carbon 

 chains, amino acids, and hexoses. 



2) Erythrite (4-hydric alcohol). 



3) Glycerine (trihydric alcohol), urea and thio-urea. 



4) Dihydric alcohols and amides of monovalent acids. 



5) Those which penetrate fastest are monohydric alcohols, 

 aldehydes, ketones, aldoximes, ketoximes, and mono, di and tri 

 halogen hydrocarbons, nitriles, nitroalkyles, neutral esters of in- 

 organic and organic acids, and many organic acids and bases, 

 and ammonia, C0 2 and the elementary gases. Also free alkaloid 

 bases and some basic dyes. 



Increasing numbers of hydroxyl groups decrease the pene- 

 trating power, and the same effect results from increasing the 

 number of NH 2 groups. Halogen substitution increases the pene- 

 trating power. The alkyl or acetyl substitution of the OH or 

 NH 2 groups increases the penetrating power. 



As a rule the basic dyes penetrate easily, and the sulphonic 

 acid dyes less easily, but there are exceptions. If the cell is 

 placed in a dilute solution of the dye the minute size of the cell 

 makes it hard to distinguish whether the dye has penetrated or 

 not, since the microscope does not concentrate the color. If the 

 cell is placed in a concentrated solution of the dye and then re- 

 turned to a colorless medium for observation, the dye that has 

 penetrated may come out again before it can be observed. Many 

 of the basic dyes are precipitated by tannin in the cells so that 

 they may be observed, or they may stain granules or gels. Some 

 dyes that do not at first become more concentrated in the cells 

 than in the medium, become more and more coarsely colloidal 

 within the cells, which constantly absorb more dye in true solu- 

 tion, until they are stained sufficiently to be observed. It is best, 

 in this case, to place the cut stem of the plant in the dye and 

 allow it to remain several days before observation. 



Overton's work has not all been confirmed. Janse states that 

 KN0 3 penetrates Spirogyra rapidly. Osterhout (1909) claims 

 that calcium and other salts penetrate plant cells at a fair rate. 

 He supposes that Overton may have failed to observe his experi- 

 ments continuously. 



