MOUNTING AND PREPARATION OF OBJECTS. 47 



Plant stems and leaves, of which sections a tenth of a 

 tniUimeter or more in thickness will serve, may be easily 

 imbedded and cut in pith. The fresh tissue, without 

 preliminary treatment, is placed in a slit cut at the end of 

 a section of moistened elder pith. A thread is tied around 

 the pith, a little below the end, to hold all together. Then 

 the pith is taken between the thumb .and forefinger of the 

 left hand, with the forefinger almost level with the end 

 and the thumb lower down. The knife or razor, held in 

 the right hand, and steadied on the forefinger of the left 

 hand, is drawn toward the body so as to cut off a thin 

 slice of the pith with its imbedded specimen. The sec- 

 tion thus cut may then be examined at once in water or 

 prepared in dilute glycerin for a glycerin mount, or 

 passed through the grades of alcohol for the balsam mount. 

 When thin sections, down to thousandths of a milli- 

 meter in thickness, are desired, the object must be im- 

 bedded in paraffin or celloidin and cut with a microtome ; 

 this is almost always necessary with animal tissues, 

 which are softer than plant tissues, and at the same time 

 tiiore opaque. For imbedding in paraffin, the object 

 must first be fixed, dehydrated, and cleared, as described 

 above. It is then placed in paraffin, kept at a tempera- 

 ture just above its melting-point in a suitably regulated 

 bath, and allowed to remain until thoroughly saturated. 

 The liquid paraffin and the object are then transferred to 

 some sort of temporary cell made of paper, or of metal 

 blocks, in which it can be cooled rapidly by a current of 

 nmning water. Slow cooling produces crystals of paraf- 

 'fin fatal to clean sections. 



