236 PHYSIOLOGY OF NUTRITION. 



when placed in water, the younger unfolded leaves becoming limp 

 first, and then the older ones. 



We now make the following very instructive experiment with 

 Helianthus tuberosus. We bend down a long shoot without sepa- 

 rating it from the plant, and without cracking it, so that a portion 

 20 cm. from the summit dips into the water contained in a 

 vessel placed below it, the summit of the stem, and the leaves 

 themselves, riot being wetted. We now with a sharp knife cut 

 through the stem below the water, so as to sever from the parent 

 a length of 20 cm. at the end of the shoot, taking special care 

 that the cut surface is not exposed to the air at all, but remains 

 throughout below the surface of the water. Our shoot keeps 

 fresh for days, while other Helianthus shoots of the same length 

 (20 cm.) cut off in the air, and then at once (say after one to t\vo 

 minutes) placed in water, rapidly wither. We may, however, in 

 various ways render them turgescent again. If we cut off a few 

 of the withered leaves, those left quickly become fresh again, since 

 the losses of the shoot by transpiration are now covered by the 

 absorption of water. A withered Helianthus shoot is fixed air- 

 tight by means of rubber tubing or a rubber stopper in the 

 shorter limb of a U-shaped glass tube containing water, so that 

 its cut end dips into the water. Mercury is now poured into the 

 longer limb of the tube. A pressure of a few cm. of mercury is 

 not indeed sufficient to revive the withered shoot, but if the water 

 is forced into the withered shoot with a mercurial pressure of 

 30-50 cm., it becomes turgescent again. With a sharp knife we 

 cut under water a length of 5 cm. from the base of a withered 

 Helianthus shoot, 20 cm. long, standing in water, taking care that 

 the new cut surface does not come into contact with the air. The 

 shoot will quickly assume a fresh turgescent appearance. 



Our experiments with Helianthus shoots, which we may repeat 

 with other plants, teach first of all the important fact that the 

 shoots wither when cut off in air and then placed in water, while 

 they keep fresh when cut off under water. There are several 

 reasons for this. When a portion of the plant is cut off in the air, 

 mucilaginous or gummy substances exuding from the surface of 

 the wound do not get removed. They are left adhering to the 

 cut surface, and so render the tissue less capable of absorbing 

 water. Moreover, the negative pressure which occurs in the ele- 

 ments of the wood of uninjured transpiring plants (see p. 182) is 

 more or less neutralised when the shoot is severed in the air, and 



