126 RADIO-ACTIVITY 



air, then those having the greatest ionising effect will have a toxic 

 rather than a beneficial effect on living organisms, while, con- 

 versely, the weaker rays will promote the function of the organism. 

 The power of ionisation being equal, then generally a long exposure 

 produces toxic effects and a brief exposure beneficial. 



This latter effect has induced experimentalists to study the 

 effect of radium on plant growth. They have found that up to 

 a certain optimum point the rays of radium act as a positive 

 stimulus. In early stages, root growth is stimulated, from which 

 there is, as result, an increased growth of shoot. Inflorescence 

 is generally fuller and is better coloured. Ultra-violet rays 

 acting for a brief period on sugar cane not only produce growth 

 but produce an increase in the percentage of sugar. If, however, 

 this optimum amount of radiation be overstepped, that is, if the 

 plant is submitted to a longer exposure or to a greater concen- 

 tration of rays, depression, culminating in complete arrest of 

 physiological function, is produced. Further exposure to the rays 

 causes death of tissue. The beneficial effects of short exposures 

 are not considered sufficiently great to justify the employment 

 of radio-active matter in horticulture or agriculture. 



Similar results have been obtained by those who studied the 

 effect of radiation on eggs and cells. For example, exposure of 

 the fertilised eggs of arbacia to rays of radium, if short, causes 

 stimulation of the cell function. If the radium is applied during 

 the approach of the germ nuclei, then cell division is accelerated. 

 If the exposure is long, cell division is retarded. The effect of 

 radium is more marked during the metaphase than during either 

 the prophase or the telophase. Eggs are not so easily influenced 

 by radium emanations after the dividing stage is passed. In order 

 to produce any effect on the rate of growth of ascaris eggs about 

 ten times the intensity of radium has to be applied as was effective 

 during the dividing stage, or the length of exposure has to be 

 increased tenfold. The ft and 7 rays seem to act on different 

 parts of the egg. The nucleus, especially if it is undergoing 

 mitosis, is influenced by the 7 rays, while the /5 radiation has most 

 effect on cytoplasm. The fertilisation membrane of nereis is 

 thickened as a result of exposure to radium. The length of 

 exposure being, in this case at least, more efficacious than the 

 intensity. 



The physician is interested in radiant energy of this type because 

 of its lethal influence on pathological growths and on bacteria. 

 Ultra-violet rays, or lights like the Simpson Light, which emit a 



