Magnus' paper is in the _Berichte der Deutschen bot. Gesellsch._, 1894, p. 39. Concerning physiological races and adapted varieties of _Puccinia_, etc., see Eriksson, "A General View of the Principal Results of Swedish Research into Grain Rust," _Botanical Gazette_, vol. 25, 1898, p. 26. For an account of Wheat-rust see Marshall Ward, "Illustrations of the Structure and Life-history of _Puccinia graminis_, etc.," _Ann. of Bot._, 1888, Vol. II., p. 215. CHAPTER XIX. SYMPTOMS OF DISEASE. _Discolorations--Pallor--Etiolation--Laying of Wheat-- Chlorosis--Yellowing--Albinism--Variegation--Uprooting, Exposure and Wilting of seedlings._ 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: _Etiolation_ is due to insufficient intensity of light, the pale sickly yellow organs being unusually watery and deficient in vascular tissue, the internodes abnormally long and thin, and the leaves generally reduced in size, or, in some plants also "drawn." Forced Endive, Rhubarb, Asparagus, and earthed Celery afford examples of etiolation purposely induced. The want of light causes the true chlorophyll colouring matter to remain in abeyance, and consequently the plant as a whole suffers from carbohydrate starvation. _Laying_ of Wheat and other cereals is a particular case of etiolation. The seeds having been sown too thickly, the bases of the haulms, owing to the etiolation and consequent lack of carbohydrates, suffer from want of stiffening tissues, and the top-heavy plants fall over. _False etiolation_ depends on a similar abeyance of the chlorophyll, but in this case due to too low a temperature. It is often seen in Wheat and other monocotyledons when the young leaves unfold in cold weather in spring. The symptoms of "drawing" and tenderness are however absent. Pallor due to too intense illumination must be kept sharply distinct from etiolation, the pale green or yellow hue being here due to the destruction of the chlorophyll by insolation, and the accessory symptoms of "drawing" are wanting.