635 LECTURE XXXVI. 



forces at work in the motile organs : in the case of the common Garden Bean, for 

 instance, the increase of the expansive force in the upper half of the organ in the 

 evening may equal a pressure of five atmospheres, so that the total expansive 

 force then amounts to a pressure of more than seven atmospheres— i. e. if we 

 suppose the transverse section of the motile organ to measure i sq. cm., a pressure of 

 more than seven kilograms would be acting on it. Of course the transverse section 

 of such an organ is usually only about i sq. mm., for which the pressure would 

 amount to only the hundredth part of the above. 



The reader will no doubt be desirous of having the theoretical statements made 

 above in the abstract, illustrated clearly by means of a few examples. I will therefore 

 next quote a series of observations which I made with Acacia lophantha. ' On the 

 20th April, 1863, a young plant of Acacm lophantha furnished with new, fine, very 

 healthy looking leaves, was placed in a wooden case, in which was also a thermometer 

 hanging close beside the plant. 



' The observations were here made hourly, but I will include in the table only 

 those observations where some change is shown, in order not to make it inconve- 

 niently long; wherever 4-10 hours are passed over in the table, it signifies that there 

 was no noteworthy alteration in that time. 



'The plant had thus for 48 hours discontinued its periodic movements all but 

 mere traces. After the last observation it was placed in the window, where, the sky 

 being cloudy, its leaflets placed themselves decidedly downwards (the angle of opening 

 being far beyond 180°) within two hours, and then appeared slight changes in position in 

 the secondary stalks. About 1 2 o'clock (noon) this Acacia suffering from rigor due to 

 darkness together with one in the normal condition were placed in the dark: the 

 leaflets of the former did not alter their position but remained open, while those of the 

 latter, on the contrary, assumed a pronounced nocturnal position within an hour» 



