774 • 



band and in other specimens weakened artificialUy the pressure of the bark 

 by cutting it longitudinally. He thus succeeded in explaining what Sachs^ 

 had already suspected, that the difference in the production of the annual 

 ring is due to bark pressure, which changes regularly in the course of the 

 year-. 



In the spring, at the time when the wood is most swollen because of 

 its absorption of water, the bark pressure is very great, as may be noticed 

 in the production at this time of new bark tears and the widening of those 

 already present. During the unfolding of the foliage, the wood loses a 

 great part of this water by evaporation. It then contracts, reducing the 

 pressure of the distended bark. This explains the recognized formation of 

 larger wood cells at this time. However, the more new wood is formed 

 under the bark in the course of the summer, the greater will be its internal 

 pressure against the underside of the bark ; at the same time the bark layers 

 lose a part of their elasticity because of drought and thus their resistance to 

 the internal pressure of the wood becomes much greater. Under such 

 increased pressure conditions, we find a production of narrow and broad 

 celled, thick-walled autumn wood. 



Another point, which I had an opportunity to observe in artificially 

 constricted places, is the increase of spiral twisting in zvood elements due to 

 the increased bark pressure. Finally, in the overgrowing of constrictions 

 made by wires, this twisting is found to be so increased that, in a certain 

 zone of the overgrowth callus, the wood cells which otherwise have a longi- 

 tudinal course lie almost horizontal. A radial section, made directly above 

 the overgrown wire ring, shows a zone of wood cells cut across instead of 

 lengthwise. These fibres, running horizontally, gradually reassume their 

 vertical, normal course when the swelling becomes less and passes over into 

 the normal trunk. 



The increased twisting of the wood elements due to increased bark 

 pressure explains also the well-known phenomenon of the non-parasitic, 

 twisted growth, which occurs especially in dry, poor soils (with Syringa and 

 Craetaegus) and has been observed in very different kinds of trees. The 

 causes of the increase in bark pressure differ in the different cases. 



The regular stratification in the wood body of the wide spring zvood and 

 narrow autumn wood thus caused is only a special case of the law proved by 

 De Vries, that an increase of the bark pressure produces narrow celled wood 

 but a loosening of the bark, on the contrary, a wide celled wood. 



It is easy to convince oneself, however, by counting the cells after an 

 artificial loosening of the l)ark, that this acts not only on the development Imt 



1 Sachs, Lehrb. d. Bot. 1st Edition, p. 409. 



2 Investigations by Krabbe, published later (Sitzung-sbericht d. Akad. d. 

 Wissensch, z. Beiiin, 14. Dez. 1882; cit. Bot. Zeit. 1883, p. 399) on the relations of 

 bark tension to the formation of the annual rings and displacement of the medullary 

 rays led to the conclusion that no effect on the annual ring formation could be 

 ascribed to the radial bark pressiu-e, on account of its insignificance. To me, the 

 method used does not seem free from criticism, so that some douiit of the correct- 

 ness of the result is justifiable. 



