PRESIDENT'S ADDRESS. Ixix. 



for the fact of leaves turning themselves to the light by the 

 presence of transparent spots, which are practically lenses 

 concentrating the rays of light, or rudimentary eyes, and which 

 tend to look, so to speak, straight at the light, so as to receive 

 the greatest amount of it. When the leaf is immersed in water 

 this twisting movement ceases, owing, the author considers, to 

 these lenses ceasing to refract the light rays, through immersion 

 in a medium of equal refractive index. Some leaves, as 

 Tropceolum, are not wetted when immersed, but remain coated 

 with an air-film, and continue their movements, unless this 

 film is removed with dilute alcohol. Such leaves, as well as 

 those with a velvet surface, would not be affected in this 

 respect by rain. The important subject of tree diseases has 

 been taken up by the Board of Agriculture, who have lately 

 issued a series of coloured diagrams, showing 45 kinds of 

 diseases. Great loss may be prevented by timely recognition 

 and attention. 



In regard to some of the lowest forms of plant life, a new 

 explanation of the movements of diatoms has lately been 

 proposed namely, that they are due to the emission of gas. 

 The idea was suggested by the very similar movements of a 

 lithia. tablet in water when giving off bubbles of gas " a sudden 

 advance in a straight line, a little hesitation, then other 

 rectilinear movements, and, after a short pause, a return upon 

 nearly the same path by similar movements." Further experi- 

 ments showed similar results. 



GEOLOGY. 



The determination of the length of periods of geological time 

 is a question which has received much attention, but which it 

 seems improbable will be ever satisfactorily determined. In his 

 address, as President of the British Association, Professor 

 George Darwin dealt with this subject, and stated that the 

 smallest possible age of the moon since its birth, based upon 



