Stewart • The Tree as an Invention 



175 



Such a branch, if its cut end is inserted 

 in an air-tight manner in a glass tube, 

 will draw a supply of water from the 

 tube with such force as to pull a 

 column of mercury up after it. This 

 demonstration, one might suppose, 

 would set us definitely ahead in the 

 solution of the problem. But here a 

 difficulty intervenes. 



The nature of the difficulty will be 

 quickly apprehended by anyone who 

 has had to learn the laws of an ordinary 

 cistern or suction pump. A suction 

 pump at its best will lift water but 

 thirt\--three feet; consequently it is not 

 advisable to install one in the third 

 story of your house. Since a column of 

 water is not strongly cohesive, and since 

 vou cannot take hold of the end of a 

 long pipeful of water and pull up any 

 quantity desired, as if it were a rope, it 

 can be lifted from above only by suc- 

 tion. The pump, by the lift of its 

 piston, removes air pressure from the 

 upper surface and tends to create a 

 vacuum, in consequence of which the 

 water is pushed up the pipe from below 

 by the weight of the atmosphere, a 

 pressure of fifteen pounds to the square 

 inch at sea level. The weight of water 

 being what it is, such pressure will 

 balance a column of thirtv-three feet. 

 No invention can be made which will 

 pull more than the laws of physics will 

 enable 'it to. And thirty-three feet falls 

 far short of reaching the top of a se- 

 quoia. 



But water has got to go up those 

 tubes to the top of a tree. This being 

 the case, scientists began to consider 

 whether water in thin columns, as in 

 these fine tubes, has not an actual 

 power of coherence, a tensile strength, 

 sufficient to stand a strong pull. Pos- 

 sibly, after all, water may be drawn up 

 from the top as if it were a rope. 

 Strange as it may seem, experimenta- 

 tion has gone quite far in proving 

 this to be the case. It seems that such 

 a column of water has a power of co- 



herence great enough to withstand the 

 pull. And the osmotic force in the 

 leaves, a strong pull of absorption, 

 might be sufficient to raise the columns 

 of water to the necessary height. Tliis 

 is the theory' that at present comes 

 nearest to satisf}'ing scientific minds. 

 But further experimentation has caused 

 more difficulty to appear. 



The rise of water to the top of a 

 tree is dependent upon evaporation. It 

 is evaporation that makes room for 

 the continual up-flow of water; and it 

 is evaporation that causes the chemical 

 concentration in the living cells which 

 gives rise to the strong absorptive pull, 

 or osmosis. This being true, one thing 

 is evident. If a plant, or a branch of a 

 tree, is placed in an atmosphere so 

 saturated with moisture that evapora- 

 tion is impossible, it will be unable to 

 keep the water flowing up its stem. 

 Experiment has shown that the intake 

 persists, though it is slowed up, even 

 when the leaves are entirely submerged 

 in water. Ever}'thing considered, we 

 mav sav that the rise of the water is a 

 mvster\', provided we do not mean to 

 imply that there is anything mystic 

 about it. 



Every cell in the top of a tree con- 

 tinues to be immersed in the life-giving 

 water. Between a cell in the sea and 

 one in the topmost twig there is no 

 essential difference of situation. And 

 the reason is that everything is done to 

 control evaporation and hold it within 

 bounds. Every leaf is coated with a 

 preparation that most effectually seals 

 it. Air can enter and water escape only 

 through microscopic openings called 

 stomates mostly on the under sides of 

 the leaves; and every stomate is capable 

 of being opened or closed according to 

 conditions. The whole trunk and e\cry 

 limb of the tree are jacketed in the pro- 

 tective, subcrized bark. There is noth- 

 ing more waterproof than bark, more 

 stubbornly impermeable. It is because 

 cork is so waterproof that it makes 



