378 



GENERAL PLANT PATHOLOGY 



and this is sometimes due to limitations of space relations. Sometimes 

 the callus is most luxuriant, as in Cuttings of Populus pyramidalis 

 (Figs. 150 and 151) and Lamium orvala (Fig. 152), which produces the 

 largest callous rolls among herbaceous plants. All organs of the plant 



Fig. 151. — Cross-section of a calloused end of a 

 poplar cutting. G, Vessel; M, pith ray. {After 

 Krister, p. 159-) 



Fig. 153. — Stem of 

 Lamium orvala with strong cal- 

 lous growth^ {After Kuster.) 



are capable of producing callus, such as roots, stems and leaves, yet 

 all parts of all plants do not have the capacity of forming it. Such 

 growth seems to reside in the living elements of exposed tissue and the 

 productive power of different kinds of tissues varies greatly. Cam- 

 bium is the most active layer in the production of callus and next to 



