202 BOTANICAL MICROTECHNIQUE 



the cover glass and slide are absolutely dry, that is, free from any glycerin. 

 The first ring should he thin and allowed to dry thoroughly before the second 

 ring is applied. More may be added if necessary. Properly sealed preparar- 

 tions will last indefinitely, but the sealing is a delicate operation and re- 

 quires some experience. 



Glycerin jelly is for some subjects as good a mounting medium as glycerin, 

 and the preparations are more durable. Transfer material from a rather 

 thick solution of glycerin to a drop of melted jelly on a warm slide, arrange 

 with needles and carefully lower a warm cover glass over the mount, taking 

 care not to inclose air bubbles. It is necessary to work quickly. The cover 

 glass may be sealed with a ring of gold size and thus strengthened. 



189. Venetian turpentine. The difficulty of properly sealing glycerin 

 preparations, together with their fragile nature, is the chief objection to the 

 glycerin mount. A method of mounting in Venetian turpentine has recently 

 been perfected by Chamberlain. ^ By this process material may be brought 

 without danger of shrinkage into a medium (Venetian turpentine) which 

 hardens like balsam and requires no sealing. The technique is somewhat 

 long and the staining methods special, but the results are striking. The 

 staining, so far as we have seen preparations, does not bring out the finest 

 details of protoplasmic structure as well as such stains as iron-alum hsematox- 

 ylin, safranin, and gentian violet. For details of this method the reader is 

 referred to Chamberlain. 



IMBEDDING IN PAEAPFIN 



190. The paraffin method of sectioning. There are several methods of sec- 

 tioning plant tissue, all of which have their limitations, because plant struc- 

 tures range from those of great delicacy, as among the thallophy tes and 

 bryophytes, to the firm and hard tissues of the sporophyte generation of the 

 pteridophytes and spermatophytes. Very firm or hard tissue cannot be cut 

 in paraffin, and sections may be made free-hand (Sec. 194) from fresh or pre- 

 served material, but are better cut in celloidin (Sec. 195). Softer structures, 

 such as anthers, ovule cases, and many developing organs of the seed plants, 

 together with the gametophyte generations of the pteridophytes and almost 

 all structures in the bryophytes and thallophytes, are sectioned most effect- 

 ively by the paraffin method. Its advantages are that sections can be cut 

 very thin, that they can easily be arranged serially on the slide, and that 

 they can be stained with greater precision. Sectioning in paraffin is pre- 

 ceded by the process of imbedding, which involves the preliminary processes 

 of dehydration and clearing, and infiltration. 



1 Methods of Plant Histology, p. 79, 1905. 



