THE STIMULATION OF PLANT GROWTH. 



17 



THE STIMULATION OF PLANT GEOWTH. 

 By Professor Henry E. Armstrong, F.E.S. 

 [Eead March 5, 1912; Mr. A. D. Hall, F.R.S., in the Chair.] 



In what does the process of growth in a plant consist — have we any 

 right to speak of stimulation of growth? Or to put the question in 

 another way : to what extent is it in our power to stimulate growth — 

 that is to say, to increase either the rate or the extent of growth ; 

 does growth in any way depend on the constant application of a 

 material stimulus, or is it merely the consequence of the presence 

 of the substances requisite for constructive purposes ? 



It is remarkable how little information the books give on such 

 points. We are aware, from experience, that manures serve to 

 promote the growth of plants, but beyond this bare fact we know sur- 

 prisingly little. I imagine it is generally thought. that manures serve 

 simply as food to plants : that they are the source of materials 

 w^hich ultimately become part of the structure of the plant. Thus 

 we add phosphoric acid and nitrogen in various forms to the soil as 

 fertilizers and suppose that these are taken up and in some way 

 built into the tissues of the plant ; potash, how^ever, is supposed to 

 act mainly in promoting the formation of starch and sugars. We 

 think of farmyard manure as valuable not only because it contains 

 potash, phosphoric acid, and ammonia, but also because it promotes 

 the retention of water in the soil and at the same time improves its 

 mechanical condition; that we need to be careful in the use of such 

 manure we know full well — but how many horticulturists are aware 

 why we need to be careful? 



A few demonstrations may help to bring some of the questions 

 I have raised to an issue. 



It is well known that though the leaf of the cherry laurel Prmius 

 Laurocerasus has no perceptible odour, though when chewed it has a 

 pleasant flavour like that of the bitter-almond, also that the crushed leaf 

 emits not only the vapour of benzaldehyde (commonly known as oil 

 of bitter- almonds) but also hydrogen cyanide (prussic acid), on which 

 account it is often made use of by young entomologists to kill insects. 

 No trace of hydrogen cyanide can be detected escaping from a sound 

 leaf hung up in a bottle together with a slip of paper soaked in an 

 alkaline solution of picrate of soda ; such paper has a bright yellow 

 colour, but when it is exposed to hydrogen cyanide the yellow 

 changes to orange and ultimately to a brick-red, the test being an 

 extraordinarily delicate one. If a drop of chloroform be introduced 

 inio the bottle containing the leaf and the picrate paper, the latter 

 soon, changes in colour, especially at temperatures near blood heat; 

 picrate paper suspended over the crushed leaf also soon becomes red. 



VOL. XXXVIII. C 



