12 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



certain dangerous fungus diseases by simply watering the plants with 

 a very dilute solution of copper sulphate. 



Others, notably Mokrzecki,^ have actually cured plants by the 

 injection of chemicals. In one experiment this author cured an apple 

 tree which was badly affected by the " yellows " or chlorosis. He 

 introduced 12 g. of iron sulphate into the stem and in three weeks he 

 found that the leaves were all healthy and dark green. In fact all 

 trace of the disease had vanished. Nor is it only possible to introduce 

 solutions by the sap current, for when teaching botany in Glasgow I 

 found it possible to introduce coloured solutions into the sieve tubes. 

 The system was not an elaborate one; I made a funnel of plasticine 

 round the stem and filled this with a coloured solution. Then with a 

 sharp knife I cut, under the liquid, into the bark not deep enough to 

 reach the wood. It was easy to show by a subsequent microscopic 

 examination that the coloured solution had travelled by the sieve tubes 

 and also entered the medullary rays. 



It would be, I think, quite possible to grossly overfeed some par- 

 ticular bloom with a specially nutritive material and perhaps produce 

 something unusual. 



But quite apart from these special surgical treatments, there is 

 surely enough evidence to show that the higher plants do sometimes 

 respond in direct answer to changes in the character of the solution 

 supplied to their roots. The root system appears to retain some traces 

 ■ of that susceptibility which is so markedly evident in bacteria and 

 various fungi. It is in fact able to adapt itself to the direct action of 

 its environment. 



But is that all that one can say on this point? It is easy to see 

 that bacteria and yeasts are immersed in their culture media and can 

 hardly help responding at once to any change in it. 



We never think of flowering plants as being similarly surrounded 

 and enclosed in their atmosphere. The beautiful researches of Black- 

 man and others have shown ns something of the various currents of 

 carbonic acid, of water vapour and of oxygen, varying both in rapidity 

 and in amount which are always entering or leaving the surface. Sun- 

 light falls upon the leaf surface, but there are all sorts of wave lengths, 

 and the intensity of the light is always changing. There may be elec- 

 tric currents of which little or nothing is -as yet known with certainty. 

 I cannot help thinking that these complex influences, always part of 

 the milieu of a living plant, are as powerful to alter the delicate balance 

 of its protoplasmic secretions, as for instance the proportion of salt in 

 the root water. 



Engine smoke, fog, ozone, carbou monoxide and sulphur fumes 

 seem at once to affect living vegetable tissues though generally in so 

 poisonous a manner that the cells have not time to adapt themselves to 

 their effects. 



* Mokrzecki, Zeit. f. Pfianzcnkranhh. Bd. 13, Heft 5. 



