324 METHODS IN PLANT HISTOLOGY 



acetic acid, 100 c.c. of 50 per cent alcohol). After thorough washing, 

 24^8 hours in running water, the blocks were softened for 3-6 weeks 

 in 50 per cent hydrofluoric acid in water. With dry material, the 

 cubes were boiled and cooled repeatedly to remove air. The usual 

 gradual processes of dehydrating, clearing, and infiltrating were then 

 followed and sections were cut with a sliding microtome, with the knife 

 placed obhquely, as in cutting celloidin. The sections are likely to 

 curl. With the forefinger of the left hand touching the section gently 

 as it is being cut, the curling can be prevented and the section sticks 

 to the finger enough to facilitate transferring the sections to the shde 

 as fast as they are cut. In this way, it is easy to cut sections 10 or 15 m 

 thick, while 20-30 m is the usual thickness of a similar section cut with- 

 out imbedding. 



The course of the vascular bundles, as they pass to the cones, is 

 quite pecuhar. Instructive preparations may be made by cutting lon- 

 gitudinal sections, about 3 mm. thick, through the apex of the stem 

 and, without staining, clearing thoroughly and mounting in balsam. 

 In this way we have mounted sections 5 cm long, 15 cm. wide, and 

 3 mm. thick. 



The course of the bundles in the xylem zone and in the cortex may 

 be traced by clearing the cubes in xylol and then transferring to equal 

 parts of xylol and carbon disulphide. Placed on a glass plate with an 

 electric light bulb beneath, the bundles are quite distinct. 



The root — Roots up to 3 cm. in diameter are easily cut freehand, 

 but cycad roots are so exceptionally easy to cut that they should be 

 imbedded in paraffin (Fig. 111). Formalin-acetic acid-alcohol is good 

 for anatomical preparations; but it is better to use the Chicago 

 chromo-acetic-osmic solution for root-tips and for the pecuhar aerial, 

 apogeotropic roots. These aerial roots, which are very common in 

 greenhouses, have a zone of Anahaena in the cortex and also have 

 bodies looking like bacteria (bacterioids). For general structure and 

 for the Anahaena, stain as for any root-tip. For the bacterioids, it is 

 better to fix in 10 per cent neutral formahn and stain as for chondrio- 



somes. 



Transverse sections of roots up to 4 cm. in diameter can be cut 

 while fresh, by flooding the root with alcohol for every section. 



The leaves. — Young tender leaves should be fixed in formalin acetic 

 alcohol and cut in paraffin. The adult leaves are rigid and can be cut 

 without imbedding. Pile leaflets, one on top of another, and tie them 



