822 Journal of Agricultural Research volxxvii.no. n 
CYTOLOGICAL STUDIES 
Nucleus and Mitotic Division. —Nem£c (n) studied the nucleus 
and division stages in the cells of the root, the stem apex, and wound 
periderm. He found noticeable differences in the formation of the achro¬ 
matic figure in the cells of the wound periderm. In the normal division 
there accumulates around the nucleus at the beginning of prophase a 
fine granular substance, in which, at a later stage, two periplasts dif¬ 
ferentiate at the poles. As these periplasts enlarge, delicate fibers 
begin to develop from the apex and grow out toward the equator, where 
they unite to form the spindle. When the spindle is formed, the nuclear 
membrane disappears and the chromatin, which during prophase formed 
a peripherally disposed thread, differentiates into chromosomes which, 
after undergoing longitudinal cleavage, arrange themselves at the 
equator. The chromosomes form short thick rods; they number at 
least 36. The nucleolus contains a single central or several vacuoles in 
which small, cyanophyllous granules are precipitated in fixed material. 
The nucleolus disappears with the formation of the equatorial plate, 
although its residue sometimes persists during metakinesis. Often they 
appear again, in the vicinity of the poles, at lie end of metakinesis. In 
the cells of the wound periderm the development of the achromatic 
figure differs primarily in that the spindle fibers are not developed from 
a hyaline periplast, but grow directly from the surface of the nucleus and 
parallel to the division axis. The nuclei of the cells of the wound peri¬ 
derm are at least 30 per cent larger than those of the normal cells, and 
the chromosomes may reach as high a number as 70. 
Since we possess in the work of Nemec an excellent account of the 
nuclear phenomena in the potato, it will suffice here to illustrate the 
principal division figures (PI. 5) and proceed to a consideration of the 
types and structure of the nuclei in the various tissues, notably the 
phloem. In the apical meristem of the young stolon all resting nuclei 
are spherical (PI. 5, A, B, and PI. 7, C), with a mean diameter of about 
12 microns. Each nucleus is composed of a large nucleolus surrounded 
by a hyaline sphere and a homogeneous nuclear substance in which 
chromatin granules of various sizes are embedded. The same type of 
nucleus, although often of larger dimensions, is found in all unspecialized 
parenchyma cells of the tuber. In the cells of the procambium and their 
derivatives the nuclei show a great variety of form and structure. 
They are often elongated with ends either blunt or pointed (PI. 5, C-G 
and PI. 7, A, B). Often they are rod-shaped, reaching a length of 
30 microns or more. Normally there is one nucleolus to a nucleus; 
in the large and elongated forms, however, two or even three nucleoli 
may be present (PI. 5, G). 
The nucleus of the young sieve tube is large and fusiform. • There is, 
as a rule, one nucleolus; occasionally one or two secondary nucleoli 
are found. As the sieve tube matures, the dense protoplasmic content, 
so conspicuous in the young element, becomes limited to a peripheral 
layer, while the nucleus itself gradually disintegrates. In the mature 
sieve tube, where the sieve plate and slime plug are easily recognized, 
a nucleus is no longer present. In a few instances the nucleus has been 
seen to persist in a more or less degenerate form even in the mature 
element. 
The protoplasm of the sieve tube initial is granular and homogeneous, 
like that of other meristematic cells. At an early period, however. 
