INJURIES TO PLANTS BY LONDON FOG, AND ENGINE SMOKE. 311 



smoke had deposited a coating of a dark grey colour. Thus Fern-fronds 

 had abnormally elongated stalks, and the stems of Pelargoniums were 

 drawn " to an inordinate degree. 

 Not only is it Assimilation which is affected, but Respiration, which 

 'does not depend upon light, as it goes on at all times, is enfeebled. 

 For this purpose, as with our own breathing, oxygen must be absorbed. 

 This, however, is impeded, owing to the faulty circulation of air within 

 the tissues ; the substances which ought to undergo further changes 

 remain incompletely oxidised. This causes an accumulation of organic 

 •acids within, and a "yellow spottiness " is visible without. 



The next ill effect of the check to light was the hindrance to Transpira- 

 tion. Experiments show that the rays which particularly favour this 

 function are red and violet. Now the violet rays are just those which 

 appear to be especially reduced. Consequently, it is not surprising that 

 the foliage became succulent and the stems of Pelargoniums soft, instead 

 of being firm and woody. To stow away the water which the plants were 

 incapable of transpiring the leaves grew coarse and large. But there is * 

 nothing to prevent the plants from absorbing moisture from the soil by 

 their roots. Consequently, as the loss of the excess by the normal 

 process of transpiration was hindered, the water accumulated within the 

 plant. This accounted for the increased succulency of the tissues. The 

 vegetative energy was thus called upon to do abnormal work, so that the 

 flowering process suffered ; not only was there a probable loss of the 

 extreme refrangible rays, which Sachs thought peculiarly favourable to 

 the flowering process, but a decided check was incurred from the excess 

 of vegetative vigour, so that the trusses of Pelargoniums were poor, and 

 often consisted of two or three flowers only. There was the additional 

 effect of starch being very imperfectly formed, as the special rays 

 required for assimilation were more or less arrested ; and since flowers 

 ■cannot be formed unless reserve food materials are present, or else there 

 be healthy foliage to make them, it was not surprising to find great 

 deficiency of blossom. 



It must be borne in mind that although certain rays may favour one 

 process, and certain others another process of plant-life, plants require all 

 the rays of the sun, and they suffer at once if anij are more or less arrested. 

 The u'orst colour is green. This is not only proved experimentally, but 

 misrht be inferred from the fact that whether a leaf be translucent or the 

 light be reflected from the surface it is green light which is thrown ofl" 

 and reaches the eye, showing that this coloured light has not been 

 absorbed, and therefore not utilised. As green glass means that all the 

 rays except green are more or less stopped and absorbed by the glass, 

 therefore just those rays only which are useless to plant-life are allowed 

 to pass through. When Decimus Burton built the Palm-stove at Kew, 

 Professor Daubeny suggested a glass tinted with green to check the glare. 

 Luckily the " tint " is so slight as to do no harm ; but it has been found 

 necessary to remove the green glass from the Fern-houses. 



The next ill effect to be considered is the result of the tarry matters 

 which not only affect trees and herbs in the open, but plants within the 

 houses. Prof. F. W. Oliver has given so elaborate an account of his 

 experimental researches that the reader is referred for details to his 



