Exercise 



LIVING CELLS (2) 11 



Strongly depending upon the substance of which 

 it is composed, and at the surface, where this 

 attraction is all directed inwardly, it produces a 

 tension which tends constantly to contract the 

 surface to a minimum. This is why such fluids 

 tend when possible to assume the spherical form 

 which presents the smallest possible surface 

 for a given volume. Any distortion from 

 the spherical form is resisted by the surface 

 tension. 



Glass is, of course, insoluble in chloroform, 

 and the introduction of a glass rod would in- 

 crease the surface of the drop. Its resistance to 

 this increase of surface is the force that tends to 

 expel the rod from the drop, or vice versa. The 

 coating of the rod with a substance soluble in 

 chloroform (e.g., shellac) entirely changes these 

 relationships, since a coated rod no longer offers 

 an incompatible surface to the chloroform, but 

 instead a substance ready to enter the same 

 phase with it. As a result, the drop now accepts 

 the glass rod. In the happy event that the size 



of the chloroform drop and the amount of 

 shellac on the rod come out about right, one 

 might observe that after accepting the shellac- 

 coated rod for a while, the drop spontaneously 

 moves apart from it again. The explanation is 

 that the drop has finished dissolving the shellac 

 off the rod and now rejects the rod itself, as 

 originally. One can think of this as a model of 

 a cell taking in an object which is partly food 

 and partly indigestible, digesting off the food, 

 and excreting the remainder. One sees also in 

 the behavior of this drop that through simple 

 forces of surface tension, the surface of separa- 

 tion between two immiscible phases (water and 

 chloroform in this case, but equally water and 

 air, or any others) forms a kind of skin with 

 special properties, an approach to a surface 

 membrane. This resists penetration by sub- 

 stances which it cannot dissolve or with which 

 it cannot react, and on the other hand it is 

 readily penetrated by substances that it can dis- 

 solve or with which it can react. 



EQUIPMENT 



Per student 



compound microscope 



slides and cover slips 



Syracuse watch glass 



piece of glass rod, 6 to 8" long, about J" thick 



6" test tube 



medicine dropper 



Per 4 students 



dropping bottle of 4% methyl cellulose 

 package of lens paper 



Per 8 students 



dropping bottle cultures of Paramecium (including 

 Paramecium aurelia Types XIII and XIV), Euglena, 

 ameba, and pond water 



dropping bottle of blue ink 



dropping bottle of carmine suspension 



dilute nitric acid (about 1 M; dilute about 60 ml of 



concentrated acid to 1 liter) 



clean mercury (10 cc) 



potassium dichromatic crystals (10 gm) 



chloroform (25 cc) 



white shellac (10 cc) 



Per laboratory 



reference books containing pictures of various 



microorganisms 



pure-line cultures of opposite mating types suitable 



for demonstrating conjugation (Paramecium bur- 



saria) can be obtained from the General Biological 



Supply House, 8200 So. Hoyne Ave., Chicago 20, 111. 



