miscellaneous: subjects for special study 477 



46. Bacterial diseases of cultivated orchids. How many? 



47. Bacterial diseases of ferns; 48. of Algae; 49. of fleshy 

 fungi. 50. Do plants harbor animal pathogens? 



51. Is loss of virulence in cultures ever due to the death of 

 an invisible symbiont? If not, why do some organisms lose 

 virulence quickly and others retain it for many years? 



SMALLER problems 



I have suggested many such problems in the preceding 

 pages and others will occur at once to teachers and students. 



in. PRODUCTION OF TUMORS IN THE ABSENCE OF PARASITES 



On susceptible species of plants, in the absence of parasites, 

 overgrowths of suitable tissues can be brought about in at least 

 five ways: (1) by introduction of irritating foreign substances, 

 that is by certain poisons, administered in stimulating rather 

 than in kilKng amounts; (2) by sHght freezing; (3) by mechanical 

 irritation, that is by woundings; (4) by over-watering in a con- 

 fined atmosphere; (5) by semi-asphyxiation with vaseline, etc. 



My attention was first drawn to this subject in 1909 as the 

 result of observations on the formation of cell-ingrowths (tyloses) 

 in the vessels of plants, especially of those attacked by bacteria, 

 and, second, by experiments in 1916 with crown-gall products. 



In the vessels of mulberry shoots attacked by Bacteriujn 

 mori, in various plants attacked by Bad. solanacearwm and in 

 old parasitized stems of Cucurbita, Citrullus and Vitis, tyloses 

 are very common but their cause has remained in doubt. They 

 occur also in many other plants but I am speaking only of those 

 in which I have studied them. To me everything goes to show 

 that they are the host-reaction to by-products of invading organ- 

 isms which may not be, necessarily, active parasites. I have 

 produced them by pure-culture inoculations with Bad. mori 

 in young shoots of the mulberry where they never occur naturally. 

 (see ''Bacteria in Relation to Plant Diseases," Vol. II, Fig. 30) 

 and also with Bad. solanacearum in very young shoots of the 

 potato (Fig. 142). Here it is certainly not the bacteria per se 

 that act as the stimulus but their soluble products, since the 

 tyloses may occur at some distance from the advancing growth 



