THE STIMULATION OF PLANT GEOWTH. 



21 



result which may be explained by the assumption that a proportion of 

 hormone in excess of the most favourable proportion is produced in 

 the soil. The balance is undoubtedly a very delicate one. 



[A series of slides which Dr. Eussell had kindly placed at the 

 disposal of the lecturer was shown, representing results obtained in 

 the case of chrysanthemums, potatos, tomatos, and wheat (figs. 7 

 and 8).] 



One very remarkable effect observed, illustrated in fig. 5, is the 

 greatly increased root growth which often takes place in sterilized soil. 



The explanation advanced by Eussell and his co-workers is that 

 soil ordinarily contains a great variety of competing protozoan and 

 bacterial organisms. On sterilization, the larger protozoa are mostly, 

 if not entirely, killed off; the bacteria are also killed but their spores 

 survive, so that new generations can arise. Normally the protozoa 

 live on the bacteria, whilst the latter live on the organic matters in 

 the soil and in living on them break them down into ammonia, car- 

 bonic acid, &c. When the bacteria are alone present in the soil, 

 therefore, greater numbers can survive and ammonia, &c., can be 

 produced in larger proportion — hence the greater fertility of the soil. 



Experiments carried out by Mr. Pickering and by Dr. Eussell 

 have also shown that germination takes place more rapidly in more or 

 less sterilized soils than in the unsterilized soils. In this case again, if 

 sterilization be carried too far, the effect may be to retard if not to 

 prevent germination — doubtless because of the production of an 

 excessive proportion of the hormone. 



The results of modern inquiry may be held therefore to show 

 that the probable answer to the question put at the beginning of this 

 address is — that growth is dependent on the constant application of 

 m^aterial stimuli, and that certain fertilizers are effective because they 

 act as stimulants. The special value of farmyard manure and of 

 organic manures generally has long been recognized ; but the reasons 

 why it is of such special value are only now being fully revealed to us. 



It is clear that many of the substances present in plants may be 

 of direct service in promoting their growth. In the case of the 

 Cruciferae, for example, the liberation of mustard oil or a similar sub- 

 stance when the glucoside present in the plant is decomposed by a 

 corresponding enzyme must furnish an effective stimulus to growth. 

 Parenthetically, it may be pointed out that the value of the raw onion 

 as a digestive stimulant, long recognized by the agricultural labourer, 

 is entirely justified by the observation that the evil smelling substance 

 it contains, which becomes so obvious when it is crushed, is a power- 

 ful hormone. 



[Specimens were exhibited showing that the Aucuba leaf is 

 blackened when exposed over Mustard paste or shredded Horseradish 

 or even over crushed Spring-onions — ^proving that these all give rise 

 to volatile hormones.] 



