386 
RAY C. FRIESNER 
Cell Division. Root tips of the species to be studied were cut from 
seedlings (or germinating bulbs) germinated at 22° to 26° C. (but always 
constant to within one degree for any particular series) in moist sawdust in 
ordinary 4-inch pots. These tips were cut at intervals of two hours, 72 to 
96 hours after the seeds or bulbs had been placed in the germinating pots. 
The tips were fixed 24 to 36 hours in medium chrom-acetic solution, washed, 
dehydrated, imbedded in paraffin in the usual way, cut into sections 10 
microns in thickness, and stained in Delafield's haematoxylin. Only those 
slides showing sections cut exactly parallel to the long axis of the tip were 
used. Two or three slides were chosen for each hour, and from each slide 
chosen the median section and one on either side were marked ofT. The 
slides, having been previously labeled with a writing diamond, were now 
given a new number without regard to the first one, and all counting of 
dividing cells was done by this last number. The two numbers were not 
compared until the entire series had been counted, so that any influence due 
to a knowledge of the time of day of the particular slide being counted was 
avoided. 
A typical observation. The slides and particular sections having been 
chosen for observation, the diameter of the section was then measured at a 
point where the root had attained a uniform diameter. Measurements 
were made by the eye-piece micrometer scale and are given accurate to the 
nearest 0.0085 mm. The diameter measured, the slide was moved by the 
mechanical stage to a point at a distance from the growing point of the 
tip equal to twice the diameter (measured where the root had attained 
uniform diameter) of the tip. The number of dividing cells in this area 
between the growing point and the imaginary line drawn across the section 
was then carefully counted. In order to facilitate the counting, a small 
rectangle was made by gluing four straight bristles (one for each side of the 
rectangle) into the eye-piece of the microscope. The section was then 
moved back and forth through this rectangle for counting. The number of 
cells dividing were recorded under the four phases: prophase, metaphase, 
anaphase, and telophase. All cells with nuclei between an evident spirem 
and the completion of the cell plate in the telophase were considered to be 
dividing. The area of the field observed was then determined by carefully 
counting the number of squares of a net eye-piece micrometer necessary to 
cover the field. This area was reduced to absolute measurement in square 
millimeters. It was soon discovered that the value so obtained very nearly 
approximated the value 7dV4, where d equals the diameter of the section in 
millimeters. The amount of difference between the two methods mentioned 
above was always very small and constant for a given species. All calcula- 
tions of areas given below were made from the latter formula. 
Since it has been shown by a number of investigators, among whom are 
Amelung (i), Sanio (31), and, more recently, La Rue and Bartlett (18), 
that in corresponding organs of plants of the same species variation in cell 
