44 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



V. General Discussion. 



The Effects of Reduced Illumination. 



I have described the chief features exhibited by plants 

 injured by fog, and to that description I have added some notes 

 on the action of certain impurities of fog. But I have designedly 

 omitted all reference to the effect which the loss of light en- 

 tailed by fog has upon vegetation. One is frequently asked by 

 cultivators of plants whether the injuries, such as are above 

 narrated, are due to poisonous substances in the air or to reduced 

 illumination. I will try to answer that question. 



It must be borne in mind that within the London area 

 cultivated plants are liable to long periods of deficient illumina- 

 tion during the season when fogs are most prevalent. For weeks 

 together during the months of November, December, and 

 January we are familiar with the dull, leaden sky, though actual 

 fog may only prevail at intervals. This state of affairs, though 

 not always by any means in an apparent manner, undoubtedly has 

 a great effect on the general health or tone of plants, particularly 

 upon those which are greedy of sunlight. 



In the first place the assimilation of carbonic acid is inter- 

 fered with. In dull weather the rate of starch-production in the 

 chlorophyll- corpuscles is very small, whilst during actual fogs 

 practically none at all is formed. Then transpiration, or the 

 giving off from the leaves of aqueous vapour, is greatly 

 diminished, falling almost to a standstill. The roots, however, 

 continue their absorption, with the result that the cells of the 

 plant become distended with water. This continued action of 

 the roots is, of course, pronounced in the case of stove plants in 

 pots ; the warmth easily penetrates the soil and keeps the roots 

 in a state of activity. Thus, in dull light, a plant comes to be 

 in a condition of being over-watered. Finally, respiration is 

 interfered with. Access of oxygen from without is a matter of 

 increased difficulty, owing to faulty circulation in the intercellular 

 passages, whilst one source of oxygen is almost entirely cut oft*, 

 viz. the oxygen which is liberated during carbon-assimilation. 



Respiration being deficient, many substances which are 

 formed in the cells are incompletely oxidised, and consequently 

 tend to accumulate. This is especially the case with the organic 



