680 



SECONDARY GROWTH IX THICKNESS 



autumn wood but abut directly against some of the vessels newly 

 formed in the succeeding spring; in other instances, the vessels of 

 successive rings are linked up by radial rows of tracheides, while the 

 two methods of connection may also occur in combination with one 

 another. In all cases communication is carried on through bordered 

 pits, and not through open pores. Gnentzsch states that the xylem- 

 parenchyma of adjacent annual rings is likewise continuous. 



Numerous attempts have been made to furnish a mechanical ex- 

 planation of the formation of annual rings. 364 Sachs and De Vries 



endeavoured to correlate the struc- 

 tural characteristics of spring and 

 autumn wood with differences in the 



tangential 



exercised by 



T.S. through the wood of the Yew [Taxus 

 baccata). g, boundary between autumn and 

 spring wood ; i, medullary rays. 



pressure 

 ~Tjfff\\ jpS^^^J&i^^^j^ L the extra-cambial tissues; but this 



ifSfl theory was refuted by Krabbe, who 

 Sp5= proved experimentally that this tan- 

 gential pressure, and consequently 

 also the compression exerted upon 

 the wood, are approximately equal 

 in spring and in autumn. Kussow 

 suggested that turgor- variations 

 might be responsible for the struc- 

 tural differences between spring and 

 autumn wood ; but Wieler showed 

 by means of plasmolytic experiments 

 that the osmotic pressure in the young xylem elements is no greater in 

 spring than in atumn. Hartig and Wieler have both attempted to 

 correlate the formation of annual rings with conditions of nutrition, 

 but in so doing have arrived at diametrically opposite conclusions ; for, 

 while Hartig maintains that autumn wood is better nourished than 

 spring wood, Wieler regards poor nutrition and an insufficient supply 

 of water as conditions which favour the formation of autumn wood. 

 Lutz accepts Wieler's view to a certain extent, since he attributes the 

 production of spring wood to a high water-content in the extra-cambial 

 tissues and in the newly differentiated xylem. The author himself 

 agrees with Krabbe and Jost, that it is at present impracticable to 

 furnish any mechanical explanation of the origin of annual rings. In 

 this respect the alternation of spring and autumn wood resembles other 

 periodic growth phenomena, the mechanism of which has so far defied 

 explanation. 



It is, of course, quite another question, whether the difference 

 between spring and autumn wood can be explained from the teleological 

 point of view 365 ; in view of the information that is available concerning 



