392 



GENERAL PLANT PATHOLOGY 



For example, the distinction between the pahsade and spongy paren- 

 chyma is often lost, because the palisade layer is not formed as such 

 and sometimes the spongy parenchyma undergoes a rich proliferation 



Fig. 159. Section uf tobaccu. Margin of infected needle wound. Tumor in 

 middle part of back parenchyma; sieve tubes at x. (After Smith, Broiun, McCulloch, 

 Bull. 255, U. S. Bureau of Plant Industry, 1912, pi. cl.) 



and red pigment sometimes appears. The same failure to form the regu- 

 lar tissues is displayed by the zoocecidia. The vascular and mechanic 

 tissues may also undergo the same reductions in cataplasm, as do the 

 assimilatory tissues, so that the vascular bundles in infected parts are 



