ABSORPTION AND MOVEMENT OF WATER 123 



and highest branchlets. The rapid ascent of so large a 

 volume of poisonous liquid is alleged to prove that living 

 cells are not necessary to the transfer. One very strong 

 objection to this conclusion is this : — although any cell into 

 which even a small amount of copper penetrates will be 

 poisoned and killed thereby, the cell next above will not 

 cease its activity untU it in turn absorbs and is poisoned by 

 the copper. Furthermore, it is well known that water, 

 which already permeates all cell-walls, will ascend faster 

 than substances dissolved in it but not permeating cell- 

 walls. The poison will ascend less rapidly than its solvent 

 because the copper-salt will be taken up by the cell- wall, and 

 will diffuse osmotically through the ceU. If then, living cells 

 do take an active part in the transfer of water, the ones 

 above and not killed by the copper can still pull up the 

 solution though to their own ultimate undoing, and they 

 Avill puU up w^ater faster than copper salt. 



Whether living cells are actively concerned in water-trans- 

 fer or not, the popular idea of the lifetime of a cell must be 

 modified somewhat. Those trees which form no "heart- 

 wood" and in which living cells may be found quite to the 

 centre (e. g. Beech and Birch), and the Palms and other 

 Monocotyledons which do not increase in thickness, offer 

 striking examples of the age attained by living cells. Stras- 

 burger* reports finding living cells in large numbers almost 

 to the centre in sections of seventy-year old beech trees. 



In the wood of trees growing in regions with pronounced 

 seasonal differences there are seasonal as well as age differ- 

 ences. The "annual rings" are divisible into so-called 

 "spring-wood" and "autumn-wood," although the latter is 

 formed long before autumn. The anatomical differences 

 which distinguish these layers of the annual ring from one 

 another are accompanied by differences in their conducting 

 power. Spring-wood is formed at the time when sap-pressure 

 is greatest, w^hen the opening of buds is followed by the ex- 

 pansion of the leaf and other surfaces from which ivater can 

 be given off, when the plant resumes all at once the activi- 

 ties which have been suspended for a season, and when most 



* L. c, p. 534. 



