414 



THE AMERICAN NATURALIST 



[Vol. XL 



(6 meters) from each other on an eighteen- meter tree, one being 

 near the ground. As a general rule the lower gauge responded 

 first, held its pressure longest, and fluctuated more. It showed 

 more pressure during the day and more suc^tion at night. The 

 higher up the gauge, other things l)eing equal, the less the pres- 

 sure. Sap conditions began first in the twigs and external layers 

 of the wood, gradually passed to the deeper tissues and lower 

 parts, ceasing again in the same order. Clark, on the other 

 hand, laid emphasis on the fact that the greatest suction as well 

 as the greatest pressure was exhibited by the gauge at the to]) of 

 the tree, but his tables show that this was very unusual. In 

 general, his upper gauge registered much below the lower and 

 fluctuated no more than did the latter. In one case Clark found 

 that on the 10th of April the upper gauge showed little or no 

 pressure while the hnver one still indicated a pressure of about 

 se\ en kilogram.-. In good seasons and good sap-spells the pressure 

 directed downward in the trunk exceeds that directed upward. 

 Later in the season the reverse may be true. Both pressure and 

 suction are greater in the outer than in the inner tissues until late 

 in the season. The outer tissues respond more (|uickly to tem- 

 ])erature changes, and the pressure changes here y)recede those in 

 the inner wood flioie 13 cm. dccj) in ;i tree (10 cm. in diameter) by 

 about mw hour. ( )ue <!cptli uii.ulit show prcssiuv or suction while 



Pressure is not readily transniiited radially, in fact tiie trans- 

 mission in this direction' is very slight indeed. The lateral trans- 

 mission of pressure is also very slight, not much more than three 

 millimeters, but diagonally it is transmitted quite readily. Two 

 years' trial in Vermont showed that a 4.5 kg. pressure muler some 

 conditions is transmitted through 244 cm. of distance. Smaller 

 pressures are not transmitted so far. It seems y^robable that the 

 pressure traverses the tissue entirely by means of the tracheav If 

 two ta})-holes are made, one in a vertical line above the other, 

 and either one is opened, the fall of pressure in the other is abrupt 

 at first l)ut after a short time ceases, and is proportional to the 

 distance between the holes. T\w remaining ])ressure represents 

 the resi-tan(e of the intervening tis.ue to tiie transmission of 

 pressure. In one case a hole bored 122 cm. above a gauge m- 



