X-rays on Plant Cells . 
219 
As produced in the original forms of ‘ gas bulb ’ these waves are hetero¬ 
geneous ; homogeneity can be obtained by ‘ filtering ’ through a suitable 
thickness of aluminium. On consideration of the more recent work upon 
light and stimulation it becomes apparent that when one deals with any 
radiation of the same type as visible light one must be prepared to consider 
both intensity and ‘ quality ’ (i. e. wave length) as important factors in the 
action upon plant cells. 
In any attempt to obtain quantitative results it is necessary to main¬ 
tain the * past history 5 of the material as uniform as possible, to expose cells 
in the same state of development, to carry on the experiments in darkness, 
and to use X-rays of homogeneous nature. 
3. The material already used in the work on electrolytes referred to 
earlier proved very suitable for examination under X-rays. When strips 
were torn from the upper surface of the petiole of Saxifraga umbrosa the 
cells were in mature condition and very constant in shape. They were 
particularly clear and easy to examine, and remained in active circulation if 
kept for as long as forty-eight hours in distilled water (free from copper), 
and for longer still when kept in tap water. The strips were sufficiently 
large for division longitudinally for certain tests where accurate control was 
needed. 
For exposure a.number of strips was mounted under a cover-slip 
i*5 in. by 075 in.; if quantitative results were needed this cover-slip was 
tested for uniform thickness by spherometer measurements. The glass 
slide was held in a wooden rack with sides to darken it, at a known distance 
from the focus spot of the X-ray bulb. The bulb itself was contained in 
the usual way in a lead-lined box with an opening only at the focus region 
so that scattered rays could not influence the material. 
For preliminary work the rays were obtained from the ordinary gas 
bulb, in which constant output is impossible. This work served to show the 
changes produced and the order in which they occurred. More accurate 
work was then undertaken with the Coolidge tube. This modern type of 
bulb gives heterogeneous rays, and the penetrating power depends upon the 
difference of potential maintained between the electrodes. The X-rays are 
produced by the action of electrons given out by a heated spiral of tungsten 
wire, and the intensity is constant if the temperature of the wire be constant. 
The heating is carried out by means of an electric current, and the strength 
of this must be maintained steady in order to give a constant temperature. 
Since the penetrating power of the rays depends upon the difference 
of potential between the electrodes, it can be tested by a 8 spark-gap ’ in 
circuit with the bulb, and the generator must be adjusted to keep its value 
constant. 
If the radiations are then passed through a centimetre thickness of 
aluminium one can obtain an output constant in intensity and quality. 
