The Laboratory 81 



appear not to have any considerable advantages 

 for plant tissue cultures. If test tubes are to be 

 used — as may be desirable in some experiments — 

 the 1" X 6" size is the most useful (Fig. 29). 



Two special pieces of glassware have been found 

 extremely useful. The first is the Maximow em- 

 bryological watch glass (Maximow, 1925, 420). 

 This has a cemented plate-glass bottom which 

 offers an optically flat surface far superior to the 

 pressed type in common use, especially for fine 

 dissections under the binocular. Those with the 

 beveled wall to the cavity are more easily cleaned 

 than those with a straight up-and-down wall. 

 This same property of optical flatness is often like- 

 wise important in the slides used for hanging- 

 drop cultures. This requirement may be met in 

 several ways. The glass Van Tiegham cell can be 

 used. Dr. Warren Lewis places low brass rings 

 dipped in paraffin on ordinary slides and cements 

 the inverted cover slips to these. A still better 

 method is to use special 3 mm. thick slides, each 

 of which is pierced with a 10-12 mm. hole. Short 

 pieces of thin (1 mm.) slides are cemented to both 

 top and bottom of these, at both ends, leaving a 

 space 25 mm. long in the center, surrounding the 

 hole, free. A 22 mm. square No. 2 cover slip is 

 fastened with vaseline to the bottom of the slide 

 in this space, forming a circular chamber with a 



