EXAMPLE 13 



Preparation of a Transverse Section of a Stem of 

 Aristolochia Stained by the Method of Johansen 



In the preparation of a transverse section of a root described in Ex- 

 ample 11, nothing was required except that an outline of the cells be 

 shown as a skeleton and that this skeleton be differentiated as to woody 

 and nonwoody tissues. 



The present example describes the preparation of a section of a plant 

 stem preserved, sectioned, and stained in such a manner as to differenti- 

 ate all of the components present. The technique to be described could 

 be applied equally well to a leaf or any other part of a plant; the stem 

 of an Aristolochia has been selected only because of its wide use in 

 teaching. 



First it is necessary to collect and fix the material. The choice of fixa- 

 tive is not of any great importance, but the majority of workers prefer a 

 "Craf" fixative, such as the formula of Lavdowsky given in Chapter 6, for 

 this type of material. About a pint of this solution should be prepared 

 immediately before it is required for use. 



Great care must be taken in the selection of a stem. If a second- or 

 third-year growth is taken, it will be too woody to cut satifactorily, while 

 a new growth of early spring will not be differentiated sufficiently to 

 justify applying a complex stain of the type used in this example. The 

 best Aristolochia material is usually obtained from the new season's 

 growth when it has partially ripened toward the end of August. Care 

 must be taken that the piece of stem is not so cut as to permit the intro- 

 duction of air into the transporting vessels since this will greatly increase 

 the difficulties of subsequent manipulation. Unless the stem can be cut in 

 the early morning while it is completely turgid, it is best to pull down a 

 branch of the vine and make the first cut under the surface of water 

 in a bucket. About a foot of stem is pushed under the water and cut off. 

 The piece should be transferred immediately to a large volume of fixative 

 and stored in the dark for about 3 days. At the end of this time the central 



222 



