I go THE PHYSIOLOGY OF PLANTS 



tissues wliich immediately surround these become brown and 

 die away. If water finds entrance to these cavities, decay 

 very soon sets in with far-reaching effect. Different kinds of 

 trees behave very differently when crown-grafted, and in 

 certain genera this operation may be undertaken without the 

 slightest danger, because they have (as, for instance, in the 

 Lime) the faculty of forming callus cells by the renewed 

 activity of pith cells. In such cases the callus will be found 

 making its way outwards through the splits, and fusing with 

 the callus formed by the cambium. 



Such an instance of complete union of stock and scion in 

 the case of a crown graft is shown in Fig. 29, which represents 

 a transverse section through a two-year-old stock of the Lime, 

 Tilia Europcca, upon which a scion of the Silver Lime has been 

 grafted. 



The operator had split the stock (Z;) very completely, had 

 cut the scion {Pf) iu such a way that some bark remained 

 attached at both sides, and had forced it right into the region 

 of the pith of the wild stock. The brown margins (s) denote 

 the boundaries between the stock and the scion. Throughout 

 the section hli stands for the old wood formed in the previous 

 year, nh for the new wood formed during the year in which 

 the grafting was performed, g for the wood vessels, c the 

 cambium layer, and mz for the normal thick-walled, un- 

 changed cells of the pith : rp is the normal parenchyma of 

 the cortex and phloem, with h the concentrically arranged 

 layers of hard bast ; re is the commencement of the callus 

 formed by the cortex. 



As soon as the scion was placed in the split, the tendency 

 to close up the wound made itself manifest. The damaged 

 cells of the cortex died off, and became separated from the still 

 living ones by a layer of cork cells. The young cortex, the 

 cambium, the young wood, and here also the outer cells of the 

 pith, commenced to divide rapidly and formed callus tissue, which 

 forced itself in between the stock and the scion. The arrows 

 in the tissue indicate the direction in which the callus masses 

 moved, and the dark lines denote the line of juncture of the 

 young healing tissues. The chief amount of this tissue is made 

 up of the callus re, formed by the cortical tissue ; but both 



