8. ANÁLISIS INTERPRETATIVO
8.6 Tejiendo las categorías
as would oocur if blocks of oells were killed In particular phases of the cell cycle while the behaviour of the remaining sells m s left undisturbed# The results do not show whether
irregularity in metaphase aeoumulation is due to wide variations in the rates of mitosis of individual cells ow partial s y n c h ra a y
of the cell cycle# Discussion
Metaphase aeoumulation is irregular in samples of the cell piqpulations of ten root meris terns of A Ilium sativum 5 or 4 days after irradiation with 250 rads# In the present experiments
insufficient roots have been scored to decide whether this
irregularity is due to sampling errors# large variations in the mitotic cycles of individual cells or partial synchrony of cell cycles# It has been shown that accumulation of mets phases in
Allium a few days after irradiation over longer periods of exposure to colchicine (2-8 hours) give values for greatly depressed rates of mitosis in all regions of the meristem except the quiescent centre# These values are comparable with those fer Viola (Clowes and Hall 1962) and Zea (Clowes 196 3 ) # Rates of mitosis after
irradiation in Zea and Viola (Clawes and Hall 1962) are based on metaphase accumulations between two fixations and it was assumed that partial synchrony of cell cycles induced by the irradiation had disappeared by the time of colchicine treatment# However# metaphase accumulation figures for Allium over short periods of exposure to oolohioine give values of T similar to or even smaller than those found in unirradiated control roots# Thus from the present results it is not possible to measure the average rates of mitosis in different regions of the meristem with any confidence# Not only is there the difficulty involved with possible partial synchrony of the cell cycles but there is
the problem of defining the meristem after irradiation# The present method gives a measure of the average rate of mitosis in the cells which constituted the meristem immediately prior to irradiation and their derivatives# However, many of these cells are no longer capable of meristem^tic activity and no allowance for such cells is made in these calculations or in those for Zea or Viei^ since all interphase cells contribute to lowering the percentage of mstaphases# There is no enpirical data on the reduction in the size of the effective meristem of roots after irradiation but it has been suggested by Gray and Soholes (l959) that about 30fc of cells in the meristem of Vicia die after a
I % f
dose of about 130 rads# this estimate seems high# but eertainly in Allium roots after 250 rads m ry cells do become meris tema tioly
ineffective either by death or by differentiation# Thus death or differentiation of cells in the meristem of Allium after 250 rads add to the difficulties of assessing rates of mitosis in discrete regions of the meristem other than the quieseent centre by an unmedified metaphase accumulation method#
However# these experiments show that a few days after irradiation# rates of mitosis in the quis scent centre rise to levels similar to those found in other regions of the meristem in unirradiated control roots and while in these regions cell cycles may be partially synchronous# in the quieseent centre cells certainly appear to be dividing a synchronously# This is another aspect of cell behaviour less affected by irradiation in the quiescent centre than elsewhere in the meristem# It is possible# that in cells of other parts of the meristem which survive
irradiation to continue active division# the length of the mitotic cycle 3 days after irradiation is similar to that 1 found in
'
unirradiated roots as Hornsey*s results for Vicia (l936) suggest# If this is so# although the 3 day peak of mitosis in the quiescent centre is a reflection of the radiation-induced mitotic delay
2-5
activily of quiescent centre cells may be associated with the reduction in the sise of the meristem racier then reduction in the rates of mitosis of cells in the rest of the meristem# but the present results do net allow such distinctions to be made*
1 ^ 0
smmARY.
1. The root meristem of Allium sativum has been clesoribed from longitudinal and transverse sections of the root apex, fk>
evidence for the presenoe of a cuiescent centre can be (joined from a Torpor - Kappe analysis of cell patterns.
2. The presence at the pole of the stake of a quiescent centre comprising 50 - 50 oells has been demonstrated by auto- rrdiographs of roots fed with tritiated thymidine#
5* Rates of mitosis in different regions of the root meristem have been measured by three independent methods ;
a) metaphase accumulation b) continuous labelling
o) pulse labelling
The rsults of the three methods are consistent and show that the rate of mitosis in the cuiescent centre is of the order of ten times that in other parts of the merlatam.
4
4. The duration of the different phases of the mitotic oyole have boon measured for various regions of the meristem by a pulse
lal)olling technique. It has been found that the time spent in mitosis and the later stages of interphase is approximately/
130
mitotic cycle in cuioacont centre cells la due to the length
of time spent in the ^>eriod of interphase prior to DNA synthesis# Quiescent centre oells contain the 2C amount of DNA during
approximately 909^ of their cell cycle,
5# It has been found tliat during recovery from oolohioine treatment the mitotic index in the quiescent centre rises while it falls in other regions of the meristem. The cells fonaing the primordlum from which the root regenerates of ter colchicine treatment are derivatives of miiesoent centre cells.
6# Using the number of cells carrying mioronuoloi as an estimate of radiation damage the radio sensitivity of different regions of tk
root meristem of Allium Sativium have been investigated after various doses of DG-rays. It has been found that the cap initials
are the most radio sensitive and quiescent centre oells by far the least sensitive# The is shorter in the cap initials than elsewhere in the meristem while it is longest in th
quiescent centre. Cells fro# the two regions of the stele are approximately equally sensitive, the averag'» values of in these too regions are the same.
7. Rates of ndtcsis in the different regions of the meristem have been measured by metaphase accimulation at various times after irradiation with 250 rads of 5&-rays# In the quiescent centre
three or four days after irradiation the rate of mitosis is greater than in the other regions of the root being ten times that found in the quiescent centre of unirradiated control roots. Average
SI
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