292 
METHODS IN PLANT HISTOLOGY 
scanty and hard to demonstrate. Iron-alum haematoxylin and 
orange will differentiate the strands if you are careful. 
Roots. —It has long been known that the root-tip furnishes- 
constantly available material for a study of mitosis (Fig. 104). 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, espe¬ 
cially Vida Faba, are very favorable. The root-tips of Trillium 
grandiflorum , Tradescantia virginica, Podophyllum peltatum, Arisaema 
triphyllum, and Cypripedium pubescens may be mentioned as known 
to be favorable; 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 Allium 
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 elongation 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 observations upon periodicity of mitosis in the 
desmid, Closterium; and in 1915, Karsten made a comparatively 
extended study of periodicity in various stems and roots, together 
with notes on algae. 
It is safe to say that the maximum number of mitoses in root-tips 
will be found shortly after noon (1:00 p.m.) and shortly before mid¬ 
night (11:00 p.m.) (Fig. 104). It is certain, however, that abundant 
mitoses may be found at other times—even at 3:00 p.m. —in sporangia 
of ferns, in anthers of angiosperms, in endosperm, and in free nuclear 
stages of the embryo of gymnosperms. 
Mitotic figures play such an important part in the development 
of the plant and in modern theories of heredity that it is worth while 
to acquire a critical technic in fixing and staining these structures. 
Use the various fixing agents—Flemming’s weaker solution, chromo- 
acetic acid with or without a little osmic, Benda’s fluid, Bouin’s 
fluid, corrosive sublimate with acetic acid, and any others. Make 
