262 



Methods in Plant Histology 



to hydrofluoric acid. Of course, veneer machines cut very large and 

 fairly thin, smooth sections from the most refractory woods. 



While a random selection of stems would furnish material for 

 practice in technic, 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; Zea Mays shows scattered 



bundles, but not the amphi- 

 vasal condition; Aloe illus- 

 trates secondary wood in 

 monocotyls; Iris has a highly 

 developed endodermis in the 

 rhizome; and Nymphea or 

 Nuphar will show scattered 

 bundles in a dicotyl. 



The sieve tubes of the phloem 

 are easily demonstrated 

 in Cucurbita Pepo, the com- 

 mon pumpkin; other mem- 

 bers of the family furnish 

 good material. Take pieces 

 of stem about 1 cm. long and 

 not too hard to cut in paraffin, 

 fix in formalin alcohol, and 

 stain in safranin, gentian- 

 violet, orange. The tropical 

 Tetracera, one of the Dillenia- 

 ceae, has sieve plates so large 





FIG. 92. Tradescantia virginica: photo- 

 micrograph of small portion of root- tip 

 chromo-acetic acid with a little osmic acid; 

 iron-alum haematoxylin. Cramer contrast 

 plate; 4 mm. objective; ocular X4; camera 

 bellows, So cm.; yellowish-green filter and 

 also a strong filter such as is used in outdoor 

 \\ork; exposure, 11 minutes. X784. 



that they are easily seen with 

 a pocket lens. The phloem 

 area is so large in the larger 

 stems that it can be cut out 



for imbedding in paraffin long after the entire stem has become 



too hard for paraffin sections. 



Roots.- -It has long been known that the root-tip furnishes 



constantly available material for a study of mitosis (Fig. 92). An 



onion thrown into a pan of water will soon send out numerous roots. 



