24 



JOUENAL OF THE EOYAL HORTICULTURAL SOCIETY. 



seedlings look strong and healthy ; sown January 9 ; those in control 

 house not germinated. 



French Beans. — A very noticeable difference for. the better in those 

 in experimental house, they and tomatos appear to require more heat in 

 house without the lamp. N.B. — I have carefully watched the ther- 

 mometer, and the mercury lamp does not raise the temperature. 



These results are very striking and cannot be attributed to forcing 

 heat. The lamp was four feet above the plants, and as these lamps 

 are remarkably free from heat rays, the direct heating effect under these 

 conditions would be practically absent. Thermometers were near the 

 experimental pots and the control pots in the neighbouring greenhouses, 

 and if anything the temperature near the controls is usually rather the 

 higher. 



But there are other ways of forcing plants. Has this method any 

 advantage to commend it? Probably it has, because though the treated 

 plants had given more rapid growth, there is no indication of over 

 extension of tissues, of lanky unnatural growth. On the contrary the 

 plants are, if anything, of a better appearance than the controls, of a 

 healthy green and a sturdy habit, looking in no wise like the unhealthy 

 product of over-forcing. 



To what is this forcing to be attributed? Presumably to the extra 

 supply of energy given to the plant in these two evening hours, and 

 apparently the ultra-violet rays have been of no harm to the plant. 



My own experiments, like those carried out by Macquenne and 

 Demoussy in France, show that too much ultra-violet radiation can be 

 harmful. Thus if plants are placed in the light of a mercury vapour 

 lamp contained within a quartz tube instead of a glass one, the quartz 

 permitting many more of the ultra-violet rays to pass, the plant soon 

 shows the effect of the treatment. 



Some plants, for instance, geraniums and Tradescantia, will wither 

 in a day or two after only an hour's exposure to the rays, others are 

 more resistant, and I have been able to grow mustard seedlings for a 

 week or two under the light before they succumbed. These mustard 

 plants were extraordinarily thick-set and sturdy and quite glabrous ; 

 living continuously under the light they had failed to produce a single 

 hair. 



This last result shows clearly how the action of the ultra-violet 

 rays is restricted to the surface of the plant, a result to be expected 

 because the rays are so rapidly absorbed by the epidermis. Prob- 

 ably the thicker the cuticle of the plant the more resistant the 

 plant will be to these rays. But when the proportion of ultra-violet 

 rays in the light has been cut down by the insertion of a clear glass 

 screen, then it is possible to grow the plants in the light without injury 

 and with accelerated growth. The conclusion seems to be that a cer- 

 tain proportion of the ultra-violet rays is not injurious to the plant, but 

 possibly even beneficial, and that in the ordinary Cooper Hewitt mer- 

 cury vapour lamp, with its luminous vapour enclosed in a glass tube, 



