348 METHODS IN PLANT HISTOLOGY 



lem, metaxylem, and the origin of secondary xylem are too soft for 

 successful freehand sections. Cut into pieces about 5 mm. long and 

 fix in formalin acetic alcohol (10 c.c. formahn, 5 c.c. acetic acid, and 

 100 c.c. of 50 per cent alcohol). Imbed in paraffin. 



The vascular system can be traced very successfully by Gourley's 

 basic fuchsin method. Take a small plant of Pelargonium {Geranium), 

 Coleus, or Tropaeolum; use only 6 or 8 inches of the tip. Cut off the 

 base — under the stain — and when the stain has reached the tips of the 

 highest leaves, fix and clear. 



For a study of woody stems, Tilia americana (bass wood) is good, 

 and shoots from 5-10 mm. in diameter are easy to cut. Very hard 

 stems like Hicoria (hickory) and Quercus (oak) must be boiled and 

 treated with hydrofluoric acid, if you expect to cut shoots more than 

 5-7 mm. in diameter. Tilia stems, up to 5 or 6 mm. in diameter, can 

 be cut in paraffin. Fix for 3 or 4 days in formalin acetic alcohol; treat 

 with 10 or 20 per cent hydrofluoric acid for 4 or 5 days, wash thorough- 

 ly, and proceed as usual. About 24^8 hours in the bath should be 



sufficient. 



Harder stems, up to 2 cm. in diameter, should be fixed in the same 

 way, treated with 20 per cent hydrofluoric acid, washed thoroughly, 

 and put into equal parts of 95 per cent alcohol and glycerin, where 

 they may remain indefinitely, but should remain at least a week. Cut 

 by the hot steam method. 



Of course, veneer machines with very sharp knives cut large sections 

 of the most refractory woods. 



While a random selection of stems would furnish material for prac- 

 tice in technique we suggest that the stem of Clintonia shows a good 

 siphonostele in a monocotyl; the rhizome of Acorus calamus is a good 

 type for the amphivasal bundle and, although a monocotyl, still shows 

 a differentiation into stele and cortex; Zea mays, universally used but 

 not characteristic of monocotyls, shows scattered bundles, but not the 

 amphivasal condition. Aloe, Dracaena, or Yucca will illustrate second- 

 ary wood in monocotyls. Iris has a highly developed endodermis in the 

 rhizome; and Nymphea or Nuphar will show scattered bundles in a 



dicotyl. 



Lenticels and tyloses are abundant and typical in Menispermum-, 

 and very thin sections can be cut without imbedding; but both these 

 structures are well developed while the stem can still be cut in paraffin 

 without previous treatment in hydrofluoric acid. 



