50 



THE SOIL AND THE SAP 



fine are they, 100 ^u, or less in dia- 

 meter, that a certain amount of 

 sap may ascend by capillarity; 

 but this would not carry it more 

 than a few inches and would cer- 

 tainly not account for the arrival 

 of the sap at the top of tall trees. 

 We shall see later that the leaves 

 are constantly giving off watery 

 vapour, and this produces a cer- 

 tain "pull" on the sap in the 

 tracheids. Now a column of water 

 offers considerable resistance to 

 being broken, as anyone can see 

 who has studied the Chain e- 

 Helice Patent Liquid Elevator. 

 There is a tensile stress which 

 keeps water in continuity; in a 

 sense water is sticky and its con- 

 tinuity is not readily broken. If 

 the sap of the upper end is pass- 

 ing away, evaporating, the water 

 at the top of the vessel pulls up 

 the water just below it, and the 

 fine column of sap in the tracheids 

 is not broken. 



The tracheids and vessels are 

 formed from dead cells whose 

 protoplasm has disappeared. To 

 prevent collapse their walls are 

 thickened, and these thickenings 

 may be spiral, ring-like (annular), 

 ladder-like (scalariform) or like 



III 



n 



IV 



Fig. 13. From longitudinal sections of the stem of a Sunflower. I, part of a 

 sieve-tube; starch grains are clustered near the sieve-plates. II, part of a 

 pitted vessel, cut in haK lengthways, showing the remains of two cross-walls; 

 the pits are shown only in a small area of the wall. Ill, a small part of a 

 spiral vessel. IV, part of a spiral vessel that was formed very early and 

 has been greatly stretched during the growth in length of the stem: the 

 spiral band has been pulled out and the whole vessel has collapsed. Magnified. 



