Page:The Physiology of the Ascent of Sap.djvu/9



T ascent of sap has been the most elusive problem in Plant-physiology. The obscurity which has surrounded the subject has been in a great measure due to the lack of adequate means of detection and accurate measurement of the rate of ascent, of transpiration, of exudation, and their induced variations. Various types of automatic recorders of great sensitiveness and precision have been devised and are described in the present work, which have been of signal service in the investigations of which an account is here given.

The result of these researches is to prove the existence of active pulsating cells throughout the length of the plant, in and from the absorbing root to the transpiring leaf. It is the pumping action of these cells that gives rise to the physiological conduction of sap, even in the absence of root-pressure and transpiration; it also injects liquid into the xylem, setting up an intra-vascular pressure with the consequent mechanical transport of fluid.

The situation of the active cells has been localised by means of the Electric Probe; the cellular pulsations concerned in the ascent of sap have been recorded by an automatic method. The invisible changes in the interior of the plant have thus been revealed, and the effect of the changes of the environment determined from the responsive variations in the pulse-record.

Other investigations are described which show that there are two distinct modes of inter-communication and inter-action between distant organs in plants: (1) the transfer of matter, and (2) the transmission of motion. The first