176 THE PHYSIOLOGY OF PLANTS 



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of the Fuchsias, the pith takes an active part in the formation 

 of the callus, and may, in some cases, form the greater portion 

 of this tissue. In Pig. 27, furthermore, m represents the 

 pith which has been cut by the knife. The groove u' is filled 

 by the callus which is growing forwards from the back ; h re- 

 presents the old wood which was formed before the cutting was 

 taken ; nh, the new wood which has been formed since then. 

 It commences with short, wide, porous, and thick-walled cells, 

 filled with starch grains, among which there soon make their 

 appearance some short reticulate vessels. These elements 

 become more and more compressed towards the periphery, and 

 resemble the normal wood more closely the nearer they are to 

 the present ring of cambium, which means the later they have 

 been formed after the cutting was taken. The cambium ring 

 arches over the cut end, and is covered on the outside by the 

 new bast and cortex, which layers are not shown in detail in the 

 figure. At the outermost portion of the cortex will be observed 

 the corky cells a, now disappearing, which were the first 

 spherical or pear-shaped callus cells which covered in the cut. 

 These rows of cells multiplied, as has been previously mentioned, 

 by the constant divisions occurring in the terminal cells. 



In that portion of the callus ring which is directed forwards 

 (ca^), and is therefore cut transversely, g represents the short 

 reticulate vessels which are the commencement of the new 

 wood. Surrounding the latter we find the cambium layer (c') ; 

 b is the old group of phloem formed before the cutting was 

 taken. It has been pushed far away from the old wood by 

 the formation of new cortical layers, and has died away at 

 its free end. Those cells which adjoined the hard bast cells, 

 however, have been liberated by the cut from the pressure of 

 the bark, and have become radially elongated (r'), while in 

 their normal condition their long axis was parallel to that 

 of the shoot. The outer layers of the old bark (r) have not 

 become changed : (0) is the oxalate of lime, which occurs partly 

 in the form of single, partly in the form of clustered crystals. 

 In some plants in which there is little tendency to produce 

 adventitious roots, the formation of callus may become so domi- 

 nant, that all the material which finds its way down the stem is 

 used for the covering layers of callus, and the formation of roots 



