150 Plant Tissue Culture 



Capillary forces will hold this piece firmly in place 

 while the 2.5 mm. distance from center to margin 

 will still permit a free interchange of gases be- 

 tween the liquid and the air in the chamber. 



Cover glasses. Standard No. 1 cover glasses 

 are usually used for hanging-drop cultures. 

 These should be thoroughly cleaned, preferably in 

 cleaning solution, rinsed, dipped in a 50-50 ether- 

 absolute alcohol mixture which is then burned off, 

 and stacked for use in a wire rack. Covers of 

 mica are frequently employed, but their optical 

 properties are seldom satisfactory and they some- 

 times contain soluble impurities which may be 

 injurious. All covers now available have objec- 

 tionable features. Cover glasses are usually made 

 of "soft" glass, are quite soluble, and tend to 

 rapidly alkalinize the nutrient drop. In animal 

 tissue cultures, where the optimal pH is about 7.6 

 and where the tendency of the tissue metabolism 

 is to render the solution more acid, this alkaliniz- 

 ing tendency of soft glass is not objectionable — 

 especially as animal tissue culture media are 

 mostly highly buffered. But with plant cultures 

 which are made in unbuffered solutions having an 

 optimal pH of about 5.4, and in which a reaction 

 above 6.0 may be very injurious, this tendency 

 may be quite serious in its effects. Pyrex covers, 

 if available, would obviate this difficulty, but these 



