354 



Edwin F. Hopkins 



Fig. 46. penetration of leaf 

 tissue. X 277 



Transverse section of epidermis. 

 (Camera-lucida drawing) 



bel ween epidermal cells (figs. 46, 47). The germ 

 tubes have not been observed to penetrate 

 directly through epidermal cells. It has been 

 noted that penetration more often occurs where 

 the conidia are more numerous. Probably this 

 is because of the greater enzymatic action, 

 which hydrolyzes the cuticular substance. 



The mycelium in the leaf, like that in the 

 bulb, is both inter- and intracellular. This is 

 shown in figures 48 and 49. After the fungus 

 has developed for a time in the leaf tissue, a 

 collapse of the cells results and causes the leaf 

 to become much thinner in the diseased area. 

 Here also, where the mycelium is still intercel- 

 lular, the cells are not killed. There is injury 

 caused in advance of the mycelium. This 

 indicates the excretion of toxic or enzymatic 

 substances by the pathogene. 

 The writer thinks that the injury caused in this disease is not due to 

 oxalic acid. Some experiments were made to determine what the nature of 

 the injury from oxalic acid would 

 be. Several plants were injected 

 hypodermically with solutions of 

 oxalic acid of various concentra- 

 tions, and lesions were produced 

 which strikingly resembled those 

 caused by a fungus. Further- 

 more, microscopical examination 

 indicated that no fungus had 

 been accidentally introduced. 

 However, the concentrations 

 were necessarily higher than 

 those produced by fungi in cul- 

 ture. The work of Brown (1915) 

 seems to show that neither oxa- 

 lates nor oxalic acid take part Fig. 47. penetration of leaf tissue, x 277 



in the toxicity of B. cinerea, but Surface view. (Camera-lucida drawing) 



