SPERMATOPHYTES—ANGIOSPERMS 
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stages showing the differentiation of protoxylem, metaxylem, and 
the origin of secondary xylem are too soft for successful freehand 
sections. Fix in alcohol-formalin-acetic acid (10 c.c. formalin, 
5 c.c. acetic acid, and 100 c.c. of 50 per cent alcohol) and imbed in 
paraffin. 
For a study of woody stems, Tilia americana (basswood) is good, 
and shoots from 5 to 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 to 7 mm. in diameter. However, with a good sharp knife and a 
rigid microtome much larger sections can be cut without resorting 
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 and, although a monocotyl 
still shows a differentiation into stele and cortex; Zea Mais , univer¬ 
sally used but not characteristic of monocotyls, shows scattered 
bundles, but not the amphivasal condition; Aloe , Dracaena , or 
Yucca will illustrate secondary 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. 
The sieve tubes of the phloem are easily demonstrated in Cucurbita 
Pepo, the common pumpkin; other members 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 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. Safranin and gentian-violet is a good 
stain for sieve tubes. It was once thought that these large sieve 
tubes afforded an obvious illustration of the continuity of protoplasm; 
but, as a matter of fact, the actual protoplasmic connections are 
