38i 



such case is treated in a study by Kissa^ on gnaii formation in Malus 

 sinensis, which he conducted under my direction. Fig. 59 shows a branch 

 of gnarl cushions, which have sprouted chiefly from the parenchymatous 

 base of a small fruit shoot. 



In cross-section, it is seen that the conical spikes represent wood 

 cylinders, of which the central tissues have arisen from broadened medul- 

 lary rays. This kind of medullary ray (Fig. 60) is either primary or is 

 produced only in a later annual ring. The wood layer of the spike is a 

 continuation of the wood ring of the mother branch. As in a normal 

 lateral axis, the spike of the 

 gnarl is covered by its own 

 bark and has also a well de- 

 veloped cambial layer. Just 

 like a normal branch, the spike 

 of the gnarl ramifies (Fig. 60 

 hm') and lengthens by apical 

 growth. But not one of these 

 axes at any time bears the 

 primordia of leaves or buds. 



The dififerentiation of the 

 tissue of the spike of the gnarl 

 takes place in the very first 

 developmental stages inside the 

 bark of the mother branch, 

 which at first appears to be 

 only swollen. This swelling is 

 produced from the upward 

 forcing of the bark by a num- 

 ber of especially strongly de- 

 veloped medullary rays, pro- 

 vided with meristcmatic tips. 

 By the further apical growth 

 of these structures, the bark of 

 the mother branch is finally 

 ruptured and the spikes of the 



gnarl, covered with their own bark, now appear as independent structures. 

 But growth in length soon ends since the bark cap and the underlying 

 meristcmatic layer dry up. Instead of an apical growth, a basal, lateral 

 sprouting now takes place in the dift'erent gnarl spikes in the interior of 

 the mother branch. 



In Fig. 60, the cross-section of a branch covered with gnarls, we see 

 that the medullary rays forming the pith of the spikes are mostly primary, 

 and, therefore, arise from the pith of the mother branch, sp indicates the 



61. Longitudinal section through the 

 spikes of a gnarl. (After Kissa.) 



1 Kissa, N. W., Kropfmaserbiklun^- bei Pirus Malus sinensis. Zeitschr. fiir 

 Pflanzenkrankh. 1900, p. 129. 



