Everybody knows in a general way when the geraniums in the window pots are drooping from want of water, or when the young Wheat is sickly, or the Pear-trees "blighted," and we have now to see how far we can systematise the knowledge that has been gained in course of time regarding the signs which sick plants exhibit.
Pallor.-Under this heading, which includes all cases where the normal healthy green colour is replaced by a general sickly yellow or pale hue, ultimately resulting in death of the parts if not arrested, we have several totally distinct diseases of the chlorophyll apparatus, each recognised by the co-existence of other subordinate symptoms. The principal varieties of pallor usually met with are the following:
The principal cases here described are dealt with in works on plant physiology, and in the works of Sorauer and Frank already referred to.
As regards damage due to uprooting of seedlings by frost, see Fisher, " Forest Protection" (Engl. ed. of Hess' Forstchutz), in Schlich's Manual of Forestry, Vol. IV., 1895, pp. 439 - 442.
On Albinism, see Church, "A Chemical Study of Vegetable Albinism," Journ. Chem. Soc., 1879, J88o, 1886.
Beijerinck's results are contained in his paper, "Ueber ein Contagium vivum fluidum," etc. (with English abstract), in Verhandl. d. Kon. Akad. v. Wetensch, te Amsterdam, 1898. Koning's paper is in Zeitschr. f Pflanzenkrank., Vol. IX., 1899, p. 65. See also Nature, Oct. 11, 1900, p. 576.