456 THE ANATOMY OF WOODY PLANTS 



of coal, which demands a transversely placed instead of an oblique 

 knife. In cutting more resistant plant substances the edge of the 

 knife must be very smooth and sharp and must be much more 

 frequently renewed than in the case of softer materials. 



It is also possible in certain cases to section unhomogeneous 

 materials, consisting of pieces of root, stem, etc., by adopting the 

 following procedure: When the sections are transverse, the pieces 

 may be clamped directly in the jaws of the wood-holder described 

 above. In the case of longitudinal sections, either tangential or 

 radial, the objects are removed from the mixture of equal parts 

 of strong alcohol and glycerin and accurately smoothed on one 

 surface by means of a sharp knife. The plane surface is dried 

 carefully with a cloth and then painted with a 6 per cent solution 

 of celloidin. After the film of nitrocellulose has thoroughly dried 

 the object is quickly dipped in 4 per cent solution and applied to 

 the surface of a block of wood which has previously been varnished 

 with nitrocellulose in the manner described in a former paragraph. 

 By clamping the block in the wood-holder it is possible to secure 

 extremely thin longitudinal sections without the labor involved 

 in imbedding in nitrocellulose. It should be emphasized, how- 

 ever, that this method is not available in the case of organs which 

 show a considerable diversity of texture. For example, the 

 branches of Ginkgo and Tsuga must be imbedded to secure success- 

 ful sections, while those of Pinus and Quercus provide good trans- 

 verse preparations without previous infiltration. 



Other vegetable materials can best be prepared as thin sections 

 without any previous treatment whatever. This is true of peri- 

 derm, as illustrated by common bottle cork and "birch bark." 

 These tissues need merely to be clamped in the microtome to fur- 

 nish sections as good as may be secured by the most elaborate 

 processes of softening and imbedding. Fresh leaves also provide 

 very thin sections when treated in the following manner: Rather 

 thick leaves are best in the case of the dicotyledons and mono- 

 cotyledons, for example, in Rhododendron and Yucca. These are 

 held between two smooth pieces of pine wood in the jaws of the 

 wood-holder. Sections are made until the knife has cut down 

 sufficiently near to the pieces of pine to secure rigidity in the leaf 



