870 



wliicli need repeating, the gnarl tubers, or tuber gnarls, can develop from a 

 dormant bud and are, therefore, originally connected with the wood body 

 of the branch. In many cases, however, they are produced as bowl-like 

 wood deposits around a group of phloem fibres, or some other bark tissue 

 group without any connection with the wood cylinder or a bud primordium. 

 The tuber is gradually pushed out into the outermost regions of the bark, 

 which is beginning to form the cortex ; the longitudinally elongated xylem 

 strands of the bark, related to the tuber formation, can press back into the 

 axial body and become elements of the normal wood cylinder of a branch. 

 External wounds in the tuber body are healed by overgrowth, just as in the 

 normal branch and there is no reason to doubt that adventitious buds can 

 develop from the overgrowth edges as well as from the normal bark of the 

 tuber, as has been stated for the olive. 



Mention should be made of the fact that the large spherical swellings, 

 produced on oak branches by the overgrowth of places where Loranthus 

 curopacus had grown, have also been termed gnarl tubers or heads. Accord- 

 ing to our division of the subject, these are not actual "gnarls" but gnarly 

 overgrowth edges. 



Tine Tammes^ describes as abnormal overgrowths the peculiar cone- 

 like processes on Fagus silvatica which usually grow broader on one side 

 and overlap. Investigation shows that the stump of a branch is involved 

 here, which has been closed by gnarly, hypertrophied wound edges. The 

 hypertrophy has been caused by the severe pruning, of the trees on account 

 of which a superabundance of plastic material is deposited at the remaining 

 centres of growth. 



Peters, in his observations on Helianthus annuus and Polygonum cus- 

 pidatum- gives an example of bark tubers in herbaceous plants. The tubers 

 produced in the middle bark should be considered as the reaction of the 

 plant to wound stimulus. A few cell groups in the bark die and dry up; 

 the cavity thus produced becomes surrounded by a cambial zone which 

 forms wood on the inner side and bark tissue on the outer. 



Th. Hartig'' mentions examples of tuber formation in roots when 

 describing the fact that young aspens occur in great numbers on cleared 

 tracts where no seed bearing trees had stood for some time. As Th. Hartig 

 explains, the little plants owe their existence to the continued growth of 

 roots left from long dead and outwardly vanished trees. 



The basis of root growth in these cases is always a tuber-like woody 

 thickening of a weak root strand. The tubers themselves are somewhat 

 like those at the gnarly base of old oaks or lindens and those in the bark of 

 the red beech ; they are the woody trunk of a dormant eye which, completely 

 individualized, lives a parasitic life on the root of the parent plant "like the 

 dormant eyes of the American species of pine." The aspen roots are kept 



1 Tine Tarnmes, tJber eigentumlich gebildete Maserbildungen an Zweigen von 

 Fagus silvatica L. Recueil des travaux hot. Ncerl. No. 1. Groningen 1904. 



2 Cit. Zeitschr. f. Pflanzenkrankh. 1905, p. 26. 



3 Loc. cit., p. 429. 



