352 METHODS IN PLANT HISTOLOGY 



and 100 per cent alcohol; stain in gentian violet dissolved in clove oil, 

 or in a mixture of clove oil and absolute alcohol, for 5-20 minutes; fol- 

 low with orange dissolved in clove oil, remembering that this will weak- 

 en the safranin and sometimes the gentian violet ; finally use pure clove 

 oil to differentiate the gentian violet. Leave the slide in xylol for 2-5 

 minutes to remove the clove oil and to hasten the hardening of the 

 balsam; 



If you use aqueous gentian violet or crystal violet, use the anilin oil 

 formula. When the safranin is satisfactory, transfer to water and then 

 to the violet. After staining in violet, dip in water to remove the ex- 

 cess of stain, and then dehydrate rapidly in 95 per cent and absolute 

 alcohol, differentiate in clove oil, and then transfer to xylol. 



The structure and development of the young root will be shown, to 

 some extent, in preparations made for mitotic figures. The origin of 

 dermatogen, periblem, plerome, and also of protoxylem, is well shown 

 in Zea mays. An ear of sweet corn, as young and tender as you can 

 find on the market, will furnish material. Cut out carefully pieces 2 or 

 3 cm. long, with two rows of grains and fix in formalin acetic alcohol. 

 When needed for paraffin sections, cut out from the grain a rectangu- 

 lar piece about 2X3 mm. and 4 or 5 mm. long; if you want to show 

 also the structure of the entire grain, take a section the entire length 

 of the grain, perpendicular to the flat side of the grain, and about 2 mm. 

 wide. Cut the latter longitudinally; the rectangular pieces are sufficient 

 for transverse sections. This is better than to try to cut out the grains 

 before fixing. 



The roots of various cereals will repay study. 



The roots of Ranunculus repens and Sambucus nigra furnish good 

 illustrations of the radial arrangement of xylem and phloem. Smilax 

 shows the radial arrangement, with a large number of poles and a very 

 highly differentiated endodermis. Cut it in paraffin. The mature root 

 will need 2 or 3 days' treatment with 10 or 20 per cent hydrofluoric acid. 

 The origin of secondary xylem and phloem is well shown in Sambucus 

 7iigra. Vicia faba is good for the origin of secondary roots. Pistia 

 stratiotes, sometimes found on lily ponds in greenhouses, and common 

 in Cuba and Southern Mexico, is unexcelled for showing the origin of 

 secondary roots. On account of the hard root cap, it needs 48 hours in 

 5 or 10 per cent hydrofluoric acid. The arrangement of cells in the 

 young roots of aquatic or semi-aquatic plants often shows a geometric 

 regularity (Fig. 123). 



