270 The Physiology of Plants BOOK in 



elements of the wood of the coniferous trees are not in 

 open communication with each other, and that the bordered 

 pits of the wood cells are not perforated but closed by fine 

 membranes, render both these views impossible. Naegeli 

 showed in 1866 that capillarity cannot supply the quantity 

 of water lost by transpiration. In 1868 Unger suggested 

 that the current travels in the substance of the walls of the 

 cells in consequence of their power of imbibition. This view 

 was at once subjected to a careful and critical examination 

 by Sachs, who claimed to have made the first definite pre- 

 sentation of it. It is particularly noteworthy as marking 

 an advance in our knowledge, inasmuch as it added to 

 the conception of the action of general physical forces only 

 held up to that time, a recognition of a special mechanism 

 differentiated by the plant itself for the performance 

 of a definite function. This basis of the idea remains 

 substantially unchanged, though Sachs' views of the mechan- 

 ism are not now considered sufficient or satisfactory. 



Sachs' investigations extended over a number of years, 

 and naturally underwent certain modifications during that 

 time. Starting with Hartig's discovery he said that not 

 only are the elements of the wood prevented from free com- 

 munication with each other, but at the time of the passage 

 of the rapid current supplying transpiration their cavities 

 are not filled with water the wood cells contain little, 

 while the vessels are empty. He calculated that the wood 

 imbibes and contains more than half its weight of water, 

 of which more than one-third is in the cell walls, they 

 being saturated with it. Investigation into their physical 

 peculiarities showed him that while they can swell very 

 little, they hold the water in a very mobile condition, and 

 consequently he said that the water must move in the 

 substance of the cell walls. He set forth this view as 

 follows in his treatise Ueber Porositat des Holzes, published 

 in 1879 : 



