Page:The Physiology of the Ascent of Sap.djvu/21



the fundamental activities in the life of the plant are the absorption of water from the soil and the conduction of the sap to all parts of the body. By them the plant obtains its inorganic food-material from the dissolved constituents of the soil, and is supplied with the water necessary to maintain its cells in that state of turgor without which its growth and various life-movements would become arrested. Every portion of a tall tree has to be supplied with water, which is absorbed by the root, conducted along the stem, and finally excreted by the leaves. Calculations have been made which show that the amount of water transpired by the leaves of a large Birch-tree may be as much as 38 kg. per day. The energy required for lifting such large quantities of water to the top of the tree must be very great, especially when, as in the giant Eucalyptus amygdalina, it attains a height approaching 450 feet (150 metres).

The problem of the ascent of sap has, from the earliest days of plant-physiology, enlisted the keenest attention of numerous investigators ; but the results obtained have not yet been found to offer any wholly satisfactory solution of it. The obscurity of the subject is, in large measure, due to the presence of numerous co-operating