The most convincing accounts, however, are since that date; see Smith, "Pseudomonas Campestris," _Cent. f. Bakt._, B. III., 1897, p. 284, and Arthur and Bolley, _Bacteriosis of Carnations_, Perdue University Agr. Expt. Station, 1896, Vol. VII., p. 17. Woods has lately shown that this disease is due to Aphides only, the bacteria having nothing to do with the disease primarily, _Stigmonose_, _Bull. 19_, U.S. Dept. Agr., 1900; but it is necessary to bear in mind that actual penetration of the cell-walls from without must be proved, as De Bary proved it for germ-tubes of fungi, before the evidence that Bacteria are truly parasitic in living plants can be called decisive. This is a difficult matter, but until it is settled we do not know whether these organisms are really parasitic in the sense that _Phytophthora_ is, or merely gain access by other means--I have traced them through dead fungus-hyphae--to the vessels, dead cell-walls, etc. The proof of infection _via_ water pores and vessels is given for one species by Harding, "Die Schwarze Faulnis der Kohls," etc., _Cent. f. Bakt._, Abh. II., B. VI., 1900, p. 305, with literature. Concerning the "Damping off" of seedlings, see Marshall Ward, "Observations on the Genus Pythium," _Quart. Journ. Microsc. Soc._, Vol. XXIII., 1883, p. 485, and Atkinson, _Bull. 94 of Cornell University Agric. Expt. Station_, 1895, p. 233. On Bacteriosis in Turnips, see Potter, _Proc. R. S._ 1901, Vol. LXVII., p. 442. CHAPTER XXVI. NECROTIC DISEASES. _Patches--Frost-patches--Bruising due to hail, shot, etc.-- Fire--Sun-burn or scorching--Sun-cracks. Dying-back--Frost-- Fungi--Wound fungi--Defoliation by insects--Defoliation by hand--Staghead._ _Necrosis._--This is a general term for cases where the tissues gradually turn brown or black in patches which die and dry up, the dead area sometimes spreading slowly and invading the usually sharply demarcated healthy tissues around. It is a common phenomenon on the more slender stems or branches of trees, especially those with a thin cortex, and the terms _Brand_ or _Scorching_ sometimes applied signify the recognised resemblance between burnt patches and these dead areas of necrotic tissue.