86i 



pressure conditions, even in the normal trunk. Our experiments in binding 

 a wire ring around a growing axis prove how much the wood fibres can be 

 forced from a longitudinal into an approximately horizontal position by 

 pressure. 



It is, therefore, the different pressure constantly endured and exercised 

 by the bark girdle, which conditions the development and course of the wood 

 fibres. Therefore, to explain gnarly wound wood, it is necessary to assume 

 a theory of the polarity of the cells and the displacement of like poles as 

 represented by Voechting and Mauled 



Bark Tubers. 



In concluding the chapter on the processes of wound healing, we have 

 still to consider the production of spherical woody swellings, or tuberous 

 outgrowths of the bark of trees and (more rarely) herbaceous plants. These 

 structures are generally called "wood tubers" or "gnarl tubers." Their 

 structure and production differ, thus necessitating a subdivision into separate 

 groups. Their character as correlative hyperplasias is their common quality. 

 They are to be considered as the counteraction of the organism to previous 

 phenomena of arrestment. The arrestment can consist in the cessation of 

 the development of a bud or, independent of any' bud, can be produced by 

 the death of scattered tissue groups in the bark. The dying of different cell 

 groups in the bark body of the woody axes occurs extensively. Frost and 

 heat, local increase in pressure and the like, can caiise the death of cell 

 groups without any injury to the whole organism, which responds, not infre- 

 quently, by an increased new formation near the centre of arrestment. The 

 dead tissue groups are sometimes only encysted by cork layers, sometimes 

 also accompanied by cell layers, increasing for some time, or permanently, 

 according to the time and kind of disturbance and the amount of the nutri- 

 tive supply in the surrounding tissue. The cell layers either produce only 

 parenchymatous protuberances or cause the formation of new wood bodies, 

 spherical in arrangement, with gnarled fibres. The latter process can 

 increase to the production of independent tuberous wood bodies within 

 the bark. 



I have made no personal study of the first group of bark tubers, the 

 production of which is traced back to bud primordia retarded in develop- 

 ment, and in consequence will quote the descriptions of earlier authors. 

 Trecul- should be named first among these. He describes in detail some 

 cases of tuber formation (in the oak and hornbeam) and comes to the con- 

 clusion that the tubers always owe their production to a bud which originally 

 is directly connected vascularly with the wood body of the branch or trunk. 

 Such a bud may lie dormant a number of years without projecting more 



1 Maule, C, Der Faserverlauf im Wundholz. Bibliotlieca botanica Part 33. 

 Erwin Naeg-ele. Stuttgart 1S96. 



2 Trecul, Memoire sur le developement des loupes et des broussins, en\asasres au 

 point de vue de I'accroissement en diametre des arbres dicotyledones. Annales des 

 scienc. nat. 3 serie. Botanique t. XX, 1S53, p. 65. 



