444 



THE AMERICAN NATURALIST 



[Vol. XL 



water through the cavity a into cell II with 10 gms. pressure if 

 a had contained only pure water. Is it not reasonable then that 

 sap would be actually forced into cell II under 10 + 10 gms. 

 pressure, and so on ? If this is true then it would seem that the 

 longer the pith ray the greater would be the peripheral pressure. 

 Large trees would be expected to show more pressure than small 

 ones. Unfortunately, records of pressure in trees of various sizes 

 have not yet been made. 



But it is not necessary or even probable that this is the case. 

 The temperature each morning penetrates the wood in a wave- 

 like manner. Only a few cells in each pith ray would be subject 

 to the critical temperature at the same time; those farther within 

 would be yet unstimulated, while those farther out would be 

 recovering from the stimulation. Therefore but few cells would 

 actually take part in the production of pressure at any one time. 

 If this is true, the size of the tree would have little effect upon the 

 pressure, within certain limits. I am inclined to suspect, how- 

 ever, that the diminished pressure in the branches and twigs may 

 be, in the main, owing to this. 



The question naturally arises why, if conditions are as here 

 outlined, a pressure of 9 kg. in the outer wood would not necessarily 

 be accompanied by a suction of 9 kg. in the inner wood, or indeed 

 by 9 kg. plus the natural suction of the tree; but no such suction 

 as this has been recorded. It may be mentioned that there seem 

 to be no recorded accounts of search for pressure or suction at a 

 greater depth than 10 to 13 cm. But the question may be con- 

 sidered from another standpoint. As stated above, the production 

 of pressure would probably be confined to a comparatively few 

 cells in each series. When pressure begins in the morning the 

 active zone is near the bark, and water would be forced into a 

 comparatively limited chamber, the contained gas would be 

 rapidly compressed, and great pressure would be produced; but 

 the water would be drawn from all the other layers of the trunk. 

 The gas of all these layers would be expanded only a very slight 

 amount, and little extra suction would be produced. Incidentally, 

 the high pressures have all been recorded in the outer wood soon 

 after flow began in the morning. 



The rising temperature probably does not act as a constantly 



