12 WILTING COEFFICIENT FOE DIFFERENT PLANTS. 



should be in close contact with the soil. If the wax does not adhere to 

 the soil surface the pots should be resealed, since the plants may buckle 

 under the wax cover if it is not adherent. 



In the case of monocotyledons we have found that the wax seal 

 can best be added immediately after planting the seeds. On germi- 

 nation these plants will grow readily through the wax, which forms 

 a perfect seal around the stems. (PL I.) In the case of dicotyledons 

 it is advisable to keep the pots in a moist chamber to reduce evapora- 

 tion until the seedlings appear above the ground, when the wax seal 

 can be applied without injury to the plants. 



Even in the case of dicotyledons this wax seal is often applied at 

 the time of planting. This prevents the soil from drying out between 

 the time of planting and complete germination. Plants which do not 

 raise the cotyledons above the soil surface and those with very small 

 seeds often push through the wax without breaking the seal. Other 

 dicotyledons push their way through the wax cover without any 

 difficulty, although it is often necessary to reseal these pots after the 

 seedlings are all up, especially in the case of plants with large coty- 

 ledons, such as the bean, squash, cucumber, etc. 



Aeration, when necessary, can readily be accomplished by making 

 two small holes through the wax to the bottom of the pot on opposite 

 sides. A glass tube drawn to a small opening and connected with 

 a wash bottle is forced into one hole to supply moist air, and a similar 

 glass tube is forced into the opposite opening and attached to an 

 aspirator to withdraw the air from the pot. The wax forms a perfect 

 seal around the glass tubes. In this way many pots can be aerated 

 and the holes resealed with a hot wire in a comparatively short time. 



The wax-seal method is also particularly adapted to the study of 

 transpiration, since by its use all loss of water is avoided except that 

 taking place through the plant tissues . This fact has been determined 

 by repeated measurements. 



(4) Serious fluctuations in temperature can be avoided by immers- 

 ing the pots to within 5 millimeters of the top in a tank of water in 

 which a circulation is constantly maintained. An apparatus for this 

 purpose which has been found very convenient is shown in figure 1 . 

 It consists of a copper tank 36 inches wide, 72 inches long, and 4 inches 

 deep, which is divided by means of a series of longitudinal partitions 

 into 10 compartments. These partitions are open at alternate ends, 

 so that a stream of water introduced at one side of the tank flows 

 back and forth the full length of the tank 10 times before finally reach- 

 ing the exit on the opposite side. The water is thus kept constantly 

 in motion around the pots. 



The circulation of water is effected by means of a small centrifugal 

 pump, as shown in the illustration. This pump takes water from the 



230 



