"VITAL" THEORIES 229 



present. The most important distinction between the wood of conifers and 

 that of angiosperms is the total absence of vessels in the former. 



The tracheids are the distinctive element in conifer wood, constituting as 

 they do the great bulk of all the woody tissues present in such species. In 

 conifers the tracheids form a densely packed type of woody tissue, composed 

 of interlocking cells. Vertically contiguous tracheids always overlap along 

 their tapering portions. Movement of water and solutes from one tracheid to 

 another is facilitated by means of the bordered pits in the adjacent walls. 

 Because of the numerous cross walls in a xylem tissue composed almost entirely 

 of tracheids, water encounters a greater resistance in moving through such 

 tissues than in traversing woody tissues which contain vessels. Nevertheless 

 it is interesting to note that the tallest trees in the world are conifers in which 

 all movement of water occurs through tracheids. 



Theories of the Mechanism of the Movement of Water through 

 Plants. — A number of different theories of the mechanism by which the 

 ascent of water is brought about in plants have been suggested, and it is prob- 

 able that more than one mechanism is involved in this process. The present 

 state of our knowledge justifies a discussion of only three possible mechanisms: 

 ( I ) that ascent of sap is caused by the "vital" activities of living cells, prin- 

 cipally those in the stem, (2) that this process occurs according to a mechanism 

 described in the "cohesion of water" theory, and (3) that upward movement 

 of water occurs as a result of "root pressures." In passing, it should be noted 

 that neither atmospheric pressure nor capillarity can account for the rise of 

 sap in plants, although both of these processes are recurringly suggested as 

 being involved in this process. Atmospheric pressure could never account for 

 a rise of more than about 10 meters, and most mature trees attain a greater 

 height than this. The capillary rise of water in tubes lO/i. in diameter is 

 only about 3 meters, and few if any of the conducting elements have as small 

 a diameter as this. 



The mechanism of the movement of water through the plant is inextricably 

 bound up with the mechanism of the absorption of water, so the following 

 discussion will necessarily treat to some extent of both of these processes. 



"Vital" Theories. — While the vessels and tracheids through which longi- 

 tudinal transit of w^ater occurs are non-living, they are always in more or 

 less intimate contact with living cells. Suggestions have therefore been made 

 from time to time that the motive power causing movement of water is fur- 

 nished by the living cells of the stem. The most recent advocate of such a 

 theory is Bose (1923). 



As an example of the "vital" theories of the ascent of sap we may con- 

 sider the proposals advanced by Godlewski in 1884. He considered that 



