MOVEMENTS OF IRRITATION. 531 



following experiment. From a bean plant growing in a flower-pot 

 we cut off towards evening, with a very sharp knife, the upper 

 half of the pulvinus of the terminal leaflet of one of the trifoliate 

 leaves. When darkness comes on, the lateral leaflets sink as 

 usual, the terminal leaflet rises, and I found, e.g., in an experiment 

 made in July (the operation was performed at 5 p.m., and the 

 plant was then placed in a dimly lighted place), that at 11 

 o'clock at night it was directed almost vertically upwards. 

 By next morning the terminal leaflet had in my experiment sunk 

 again. It follows from this that in leaves which exhibit move- 

 ments of variation, the evening change of position does not take 

 place because the turgidity of the cells of one only of the opposed 

 halves of the pulvinus (in our case the upper) is intensified 

 in darkness. On the contrary, darkness raises the turgidity of 

 all the cells of the parenchyrnatous mantle of the pnlvinus, 

 while exposure to the light reduces it; but this elevation or 

 depression of the turgidity does not proceed at the same rate 

 in the opposed halves of the pulvinus. In the pulvini of the 

 bean leaflets, for example, the turgidity augments more rapidly 

 in the cells of the upper half than in those of the lower ; hence 

 the leaves bend downwards. But that the turgidity does at the 

 same time increase in the lower half of the pulvinus is clear 

 from our experiment, since if this had not been the case, the leaf 

 operated upon would have been unable to raise itself in the 

 evening. 



Some further experiments may here be made which teach that 

 the movement-joints of the bean and those of other plants are 

 similar in their behaviour are geotropically sensitive. 



We grow beans in flower-pots in bright diffuse light. When 

 the plants have formed a few trifoliate leaves, we lay strips 

 of wood over the soil in the pots to prevent it from falling out, 

 invert the plants, and leave them in the inverted position exposed 

 to the light (best in the light shade of a tree). It is of advantage 

 if the plants are allowed to grow from the beginning in the 

 light shade of a tree. If the temperature is high we can proceed 

 to observations on the inverted plant even at the end of six to 

 eight hours. Before inverting we had measured the angle formed 

 with the shoot axis by the stalks of the primordial leaves, as 

 also of the trifoliate leaves. We had likewise determined the 

 angle made by the blades of both kind of leaves with the leaf- 

 stalks. If we repeat these determinations after the plants have 



