SPERMATOPHYTES— ANGIOSPERMS 349 



The sieve tubes of the phloem are easily demonstrated in Cucurhita 

 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-acetic-alcohol, and stain in safranin, 

 gentian violet, orange. Beautiful sections of the sieve tubes and sieve 

 plates can be obtained by cutting out, very carefully with a thin safety 

 razor, a single vascular bundle with a little of the surrounding paren- 

 chyma, and imbedding it in paraffin. 



The tropical Tetracera, one of the Dilleniaceae, 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, practically free from 

 the xylem, and imbedded in paraffin. 



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 scanty and hard to demon- 

 strate. Iron-alum haematoxyUn and orange will differentiate the 

 strands if you are careful. 



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

 stantly available material for a study of mitosis (Fig. 122). An onion 

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

 beans in water for several hours and then plant them in loose, moist 

 sawdust. In a greenhouse, with "bottom heat," the primary root will 

 be long enough in 2 or 3 days. The large, flat beans, especially Vicia 

 faba, are very favorable. The root-tips of various species of Trillium, 

 Tradescant'ia virginica, Podophyllum peltatum, Arisaerna triphyllum, 

 and Cypripedium puhescens may be mentioned as known to be favor- 

 able; but it is very possible that the best root-tip has not yet been 

 tried. 



Cell division does not proceed with equal rapidity at all hours of the 

 day. Kellicott has shown that in the root-tips of Alliu77i there are in 

 each 24 hours two periods at which cell division is at the maximum, 

 and two at which it is at the minimum. The maximum periods are 

 shortly before midnight (11:00 p.m.), and shortly after noon (1:00 

 P.M.). The minima, when cell division is at the lowest ebb, occur about 

 7:00 A.M. and 3:00 p.m. When cell division is most vigorous, there is 

 little elongation, and when cell division is at the minimum, cell elonga- 

 tion is at the maximum. Consequently, root-tips of Allium should be 

 collected about 1 :00 p.m. or 11 :00 p.m. Lutman, later, made observa- 

 tions upon periodicity of mitosis in the desmid, Closterium; and in 



