16 



The cells of each species of plant produce under normal conditions their own 

 particular substances which prevent their infection by all fungi except those that 

 have the power of overcoming these substances. If, however, a plant becomes 

 affected by injury, or by unsuitable surroundings, such as heat, light, water, etc., so 

 that its vitality is weakened, it may be unable to produce the anti-bodies necessary 

 to resist the entrance of certain fungi, and thus becomes susceptible to disease. 



Some recent studies by Dr. M. T. Cook, of Delaware Agricultural Experiment 

 Station (Bulletin 91), go far to show the important part played by tannin in plant 

 cells in warding off fungous attacks. This substance is a very common constituent 

 of most plants, and was for a long time considered as a waste product. Pfeffer, 

 the great physiologist, however, says that it is undoubtedly produced for a definite 

 purpose, and is not a mere by-product under all circumstances. Tannin is abundant 

 in the bark, and in the coats of the fruit and seed, and quite conspicuous in injured 

 parts of plants, especially in galls produced by insects and fungi. 



The presence of tannin in plant tissues may have some bearing on immunity. 



Von Schrenk (Bulletin 14, Bureau of Plant Industry, U. S.) says : " Wherever 



organic food is stored bacteria and fungi are sure to grow with great rapidity if 



moisture is present. This is one of the reasons why sap wood rots so much faster 



than heartwood. It (heartwood) is further protected by antiseptic bodies, such as 



various resins, the tamiins, etc." 



« 



Dr. Cook in his recent Bulletin (Delaware, No. 91) shows that " tannin has 

 a tendency to retard or inhibit the growth of fungi," that '' it is the tannin and not 

 the cell wall that is most important in checking the growth of mycelium," that 

 " some spores are more resistant than others of the same species," and that " par- 

 asitic forms are more sensitive to the action of tannin than the saprophytic forms." 

 Dr. Cook concludes : " It appears that tannin is an important factor (in chem- 

 otactic action) and that its importance varies in accordance with the other sub- 

 stances with which it is associated in the cells of the host plant. While tannin no 

 doubt serves as a protective agent, its efficiency in this direction will vary somewhat 

 with the character of the other substances within the cell. This may account for 

 the variation in power of resistance between species, varieties, and individual plants." 



There is, however, no reason to beheve that tannin is the only chemotactic 

 substance in the cells of the plants. Both organic and inorganic acids act repul- 

 sively towards fungi. Generally speaking, ammonium compounds, phosphates, 

 peptone, asparagin and sugar are good attractive agents. 



The degree of concentration of the solution is also an important factor in 

 chemotaxis. 



