MR JOHN AITKEN ON DEW. . 59 



the tissues of the plant, and then producing an internal pressure, which forced 

 the sap to escape by the exuding pores. But what is the condition of the 

 tissues during day when transpiration is going on % An answer to this was easily 

 obtained by removing the receiver from the plant, and allowing evaporation to 

 proceed from its leaves. The result was that the pressure inside the plant fell, 

 and a negative pressure took its place ; the mercury first fell in the U tube, and 

 then rose on the other side. The mercury was drawn up in one case to a height 

 of 140 mm., and in another plant to 180 mm., the height seeming to depend on 

 the rate of evaporation, and the perfection and closeness of the tissues of the 

 plant enabling it to stand a greater or less pressure before air forced its way 

 inwards. We see from this that exuding plants during night, and at times when 

 there is little evaporation, have an internal pressure tending to distend their 

 tissues, and have a negative or external pressure during the clay tending to 

 press the tissues inwards. This internal pressure may help to explain the more 

 rigid appearance of the leaves of plants at night ; while the negative pressure 

 or degree of vacuum produced inside the leaves by transpiration explains 

 the manner in which water is taken up by cut flowers and branches of plants 

 when their ends are placed in it, and it also explains something of the peculiar 

 curving of leaves when withering. 



I have said that the pressure above measured inside of the cauliflower plant 

 would have been much greater if the exuding pores had been less in size or 

 number — that, in fact, the pressure then measured was not the maximum root 

 pressure. To test this point, the plant was now cut across near the bottom of 

 the stem, within two or three centimetres of the root, so removing all the leaves 

 with their exuding pores ; and the pressure gauge was attached to the stem 

 near the root. The gauge now rose very rapidly, and in a short time indicated 

 a maximum pressure of 760 mm., the india-rubber connecting tube requiring to 

 be strongly bandaged to prevent it bulging. It seems strange that the delicate 

 tissues of a young plant should be able to produce and resist so great a pressure. 

 We must however remember that this last registered pressure is one to which 

 the plant is never subjected when under natural conditions, but even the 290 

 mm. measured when the plant was exuding freely does seem a great pressure 

 to exist in plants. 



A poppy tested in the same way showed an internal pressure of 175 mm. 

 with its leaves all on, and exuding freely in saturated air. This lower pressure 

 compared to the cauliflower, would seem to indicate that the exuding pores are 

 larger or more numerous in the poppy than in the cauliflower, as the former are 

 fully as wet as the latter on dewy nights. When the poppy was cut across, 

 and the gauge attached to the main stern near the root, the pressure rose 

 to as much as 1040 mm. 



The pressure inside grass has not been easily measured, owing to the diffi- 



