60 MR JOHN AITKEN ON DEW. 



cult}- of making a tight joint with the gauge, and so delicate a structure as a 

 grass stem. The highest pressure observed before the joint gave way was 160 

 mm. The measurement was taken with all the blades on, and exuding freely in 

 saturated air. This pressure is just a little less than was found in the poppy 

 when under similar conditions. No measurements have been made with all 

 exudation stopped, on account of grass not growing in a form suitable for 

 making a measurement of this kind. 



The following are a few readings of the maximum root pressure given by 

 different plants. These plants were small and still in the seed-bed, but too 

 large for transplanting. 



Cauliflower, .... 875 mm. 

 .... 920 „ 



.... 1065 „ 



Cabbage, .... 1310 „ 



From the above figures it will be seen that the pressures given by the 

 different cauliflower plants varied considerably ; and that the cabbage experi- 

 mented on was capable of exerting a root pressure equal to forcing its sap to 

 a height of about 58 feet, thus showing an extraordinary reserve of energy. 



It was shown in the first part of this paper that the leaves of these plants 

 exuded when hydrostatic pressure was applied to their stalks, and we have 

 now shown that there is abundance of pressure inside these plants at night to 

 produce the exudation. Nothing like an attempt, however, has been made here 

 to give either the exact pressure inside different kinds of plants or the pressure 

 under different conditions. It has already been stated that the conditions 

 affecting the pressure are much too varied for the figures to be settled by a 

 few experiments ; all that has been attempted is to show that in exuding plants, 

 there is abundance of pressure to produce the results claimed. 



In connection with this subject, it was interesting to notice the variation in 

 the exudation of grass during the late continuance of dry weather. While the 

 soil was clamp exudation went on as usual, but when the ground got drier the 

 exudation gradually got less and less, and at last it entirely ceased. Even when 

 the grass was covered with an enclosure, and surrounded with saturated air, 

 no exudation took place, and yet the grass was green and growing ; it took 

 some time after the grass was wetted before the activity was great enough to 

 give rise to exudation. During this dry weather, while the grass had ceased to 

 exude, it got moist at nights with the hot vapour rising from the ground, the 

 lower parts, particularly where exposed to radiation, being wetter than the tops 

 of the blades. 



It is very difficult to get an idea of the number of plants that exude, so 

 much depends on the vitality of the plant at the time, and on the amount of 



