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A TEXTBOOK OF BOTANY [Ch. IV, 12 



pear on old Elms, especially near the bases of the lower 

 great branches, are composed of complexly contorted and 

 twisted masses of wood, often beautifully grained when 

 sectioned and polished. They are formed by areas of cam- 

 bium, which, instead of keeping their places and parts in the 

 regular fibro-vascular cylinder, proceed to grow profusely, 

 and thus are thrown out into irregular folds. A less extreme 

 case is found in Curly Birch, and in some other irregularly 

 grained hardwoods highly valued in fancy carpentry. In 



some cases such growths 

 are apparently started by 

 injurious strains, which 

 would explain their fre- 

 quency at the bases of 

 great branches ; and very 

 likely they represent areas 

 in which the growth-con- 

 trol mechanism has been 

 ruptured by the strain. 

 It is interesting to note 

 that a close analogy exists 

 between these burls and 

 the troublesome tumors 

 which form in the human 

 body, for the latter also 

 are formless growths re- 

 sulting from continued operation of the growth energy of 

 the tissues after the control stimuli have been inhibited, 

 usually as result of some strain or other accident. Other 

 burls, however, with various kinds of knotty growths, are 

 started by presence of parasites, which also inhibit the 

 usual control, presumably by chemical action. Of this 

 nature is the remarkable " wooden flower," sold to tourists 

 in tropical America (Fig. 148). It is nothing but a stem in 

 which a parasite has inhibited the growth control over a 

 limited area, leaving that part free to grow as it happens. 



Fig. 148. — A Wooden Flower, or 

 Wooden Rose, on a leguminous plant ; 

 X 4. The parasite which induced it was 

 a flowering plant, Phoradendron. (From 

 Engler and Prantl, Pflanzenfamilien.) 



