180 MINNESOTA BOTANICAL STUDIES. 



with results almost, if not equal, to those obtained when it was 

 passed first into 20 per cent, glycerine. When the sections were 

 removed from the knife they were placed in 20 per cent, glycer- 

 ine, and from this into 50 per cent, and then into absolute glycer- 

 ine ; or from 20 per cent, glycerine to water and then through the 

 alcohols to an alcohol stain or directly into a water stain. The 

 freezing device mentioned above proved very satisfactory in 

 many respects. The tissues can be frozen in a minute's time, 

 are held firmly in place, and the gum arabic, when of the proper 

 consistency, is an excellent imbedding medium. The difficulty 

 experienced with this method of cutting such delicate tissues as 

 are found in many of the seaweeds lies in the handling of sec- 

 tions after they are cut. Especially is this true when it is desir- 

 able to stain the sections and when they must be transferred 

 through several per cents, of alcohol and glycerine. With such 

 tissues as are found in the frond of Constantinea^ where they 

 part with great ease, the difficulty is augmented, and it was 

 found almost impossible to preserve thin sections whole. An 

 attempt was made to obviate this difficulty by mounting the sec- 

 tions directly from the microtome knife into glycerine jelly at a 

 temperature sufficient to keep the jelly semi-fluid. This was in 

 part an improvement, but necessitated the mounting of many 

 worthless sections. When sections unstained were mounted in 

 glycerine or glycerine jelly, the great transparency of the 

 swollen cell walls added a new difficulty. 



Material was also passed into paraffin and stained on the slide 

 and mounted in Canada balsam by the usual methods. Ex- 

 treme care was found necessary, on account of the delicacy of 

 the tissues, to make the stages from one fluid to another by very 

 gradual changes. I was unable to prevent a partial shrinkage 

 of the cell contents. Sections by this method were otherwise 

 quite satisfactory, having the advantage of use in serial work. 

 Staining is necessary in this method since the sections become 

 almost invisible in Canada balsam. 



Sections cut freehand with a razor or in a hand microtome 

 with the material imbedded in pith have furnished most of the 

 material from which the accompanying plates are drawn. Suf- 

 ficiently thin sections were obtainable in this way with the great 

 advantage of certainty as to the normal condition of the tissues 

 and of speed in preparation. A large number of such sections 

 can be cut and preserved in 2 per cent, formalin for a long time 

 and are ready for use at any moment. 



