PLANT DISEASES. 



727 



already stated, to a lack of knowledge of the life-history of fungi, and 

 consequent inability to check the disease, or even to prevent its com- 

 mencement ; whereas the possession of this knowledge would, in many 

 instances, render such a course possible. 



By the uninitiated the appearance of a fungus bursting through the 

 tissues of a leaf, stem, or fruit is quite naturally considered as the earliest 

 appearance of the disease ; and this idea is strengthened by the fact that 

 in the majority of instances, up to the moment of this rupturing of the 

 tissues and the appearance of the fungus on the surface, the plant 

 appeared to be in perfect health. 



Now the above line of reasoning, in spite of appearances in its favour, 

 is radically wrong from beginning to end. 



The usual course followed by a parasitic fungus is illustrated by the 

 following examples. 



The flowers of Scilla hifolia and Cliionodoxa Liicillea are often dis- 

 figured by the presence of a mass of black powder produced in the 

 anthers. This powder consists of the spores of a minute parasitic 

 fungus, and w^hen quite ripe is scattered over the entire flower, completely 

 destroying its beauty. 



The appearance of this sooty mass in the flower is considered by the 

 gardener as the first indication of the fungus in the plant. For a long 

 time it w^as supposed, even by scientific men, that the anthers were 

 infected directly by fungus spores floating in the air. Now that the life- 

 history of the fungus is known, its mode of attack is found to be very 

 different from what was expected. The anthers are not directly infected 

 by the fungus spores, but w^hat happens is as follows. The ripe spores 

 produced in the anthers are carried into the soil by rain, where they 

 germinate, and if the mycelium of the fungus happens to come in con- 

 tact with a very young seedling Scilla, it enters the stem, and continues 

 to grow in the tiny bulb of the young plant. It is important to 

 remember that the Scilla or Cliionodoxa can only be infected by the 

 fungus while it is quite young. When it is a month old all danger is 

 past. When the Scilla is old enough to bloom, the mycelium of the 

 fungus present in the bulb grows up along with the flowering stem, 

 passes into the anthers, and in due course produces a sooty mass of 

 spores ready to infect other seedlings. Year after year, w^hen a plant is 

 once infected, the mycelium of the fungus, living in the bulb, passes up 

 into the flower to form its spores ; yet the plant is not killed by the fungus 

 present in its tissues, and there is no outward indication of the parasite 

 during the growth of the plant. 



The anthers of many other plants are also attacked by " smut " or 

 fungi belonging to the genus Ustilago. In all such that have been 

 examined, the mycelium of the fungus is found to be perennial in the 

 root of the plant, and in all cases the plant can only be inoculated while 

 in the earliest seedling state. Many kinds of caryophyllaceous plants. 

 Carnations, Pinks, Silcne, Lychnis, &c., are subject to this disease. 



The life-history of the fungus forming " smut" in the grain of Oats 

 is similar to the above ; the plant can only be infected in the seedling 

 stage, the fungus growing along with the stem, and not betraying its 

 presence until the spores form a black sooty powder in the grain. (Fig. 304.) 



