206 TRANSPIRATION. 



269. Negative Pressure in the Water Channels. 



In the stem of an actively transpiring plant there is often 

 a partial vacuum in the water-conducting channels of the 

 xylem. Starting with a plant in which these channels 

 (vessels or tracheids or both) are filled with water, if the 

 plant does not receive a good supply the water gradually 

 disappears from the cavities of these xylem elements, so 

 that they will contain moist rarefied air, thus leading to 

 " negative pressure." If the stem is cut across under 

 liquid this at once rushes into the vessels until the normal 

 (atmospheric) pressure is equalised ; if the stem is cut in 

 air, the air of course enters the opened vessels, to equalise 

 the atmospheric pressure. 



In connection with negative pressure, it is to be noted 

 that the presence of air in the vessels retards the trans- 

 piration current, when the air is under ordinary pressure ; 

 also that, although the membranes of the vessels (and 

 tracheids) are very permeable to water, they are much less 

 permeable to air when moist, but when dry they allow air 

 to pass readily through them. Negative pressure is also of 

 practical importance in experiments on transpiration the 

 stem or petiole should be cut under water, not in the air. 



It is easy to demonstrate the existence of negative pres- 

 sure in any shoot in which the leaves are transpiring and 

 the root is not absorbing enough water to replace that lost 

 by transpiration. 



(a) Pull up two Bean seedlings, and let them lie on the 

 table until somewhat wilted ; then place each seedling 

 with the lower part of the shoot in red ink, and cut across 

 the stem under the ink. That the ink at once rushes up 

 in the xylem of the bundles is seen by immediately slitting 

 the stem of one seedling longitudinally, and cutting trans- 

 verse sections at different heights in the other seedling. 

 For comparison, pull up a third seedling which has been 

 well watered and is not at all wilted, treat it in the same 

 way, and note that in this case the ink travels slowly up 

 the stem. 



(6) Of two similar herbaceous plants in pots, keep one 

 unwatered and in a dry place, the other being kept moist 



