532 KELLICOTT: PERIODICITY IN ROOT OF ALLIUM 
a distance of 5 micrometer spaces (eye-piece micrometer ruled in 
millimeters, ocular 3, objective Zeiss A, tube-length 140 mm.) 
from the growing point of the tip (the root has 
a attained a uniform diameter at this point) its diam- 
d | eter was measured carefully. An imaginary line 
De - 2 was then drawn across the section at a distance 
+ from its extremity of twice its diameter. The area 
examined was that below this transverse line 
[t] (FIGURE I), and was found in every instance to 
contain practically all of the actively dividing 
cells of the undifferentiated tissue. 
The number of dividing cells within the cor- 
Ficure i. Dia- responding areas of the three selected sections was 
a 12 ele aia then carefully counted. The dividing cells were 
DD iametersspace tabulated in three groups. The first group in- 
from apex cluded those cells in the early stages of mitosis 
‘ ave extend up to the time of the formation of the equatorial 
tip — 15. plate, 7. ¢., all stages of the prophase. The sec- 
ond group included those cells in metaphase and 
early anaphase up to the time when the divergence of the chromo- 
somes is complete. The last group included the cells in late ana- 
phase up to the time of the reconstruction of the nucleus and 
formation of the new cell-wall. These groups were then added 
and the average of the three sections taken as the number char- 
acteristic of the tip. 
In order to avoid error due to the comparison of tips of differ- 
ent sizes the areas of the sections were reduced to a common area 
and the proportional numbers of dividing cells calculated. The 
average area of the sections was about fifteen micrometer squares > 
therefore the numbers of cells in sections having larger or smaller 
areas than this were calculated proportionally. 
This method of comparison is justified by the fact that the size 
of a part or organ is determined by the number of cellular elements 
contained in it and not by variations in their size.* Several counts 
were made in given areas of large and small root-tips of Allium, 
and this was found to be true. 
4 
‘ 
* Amelung, E. Ueber mittlere Zellengrésse. Flora, 77: 3 6 6. 1893. sus 
burger, E. Histologische Beitrige. 5: 117-118. 1893. 
