09 June 1972
Vol 176, Issue 4039

About The Cover

Cross section of pin oak wood (about X 5.5). A column of water supported only by atmospheric presurre can be only 10.3 meters high. Since trees grow higher than this, the acent of sap in the xylem must be due to another machanism. Tension has been assumed to be the machanism, but a new theory is proposed. See page 1129. [From patterns of life: The Unseen World of plants; copy-right © 1966 by William M. Harlow; reproduced by permission of Harper & Row]