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CHAPTER II. 



Vegetative Organs — Mycelium. 



Among parasitic fungi, such as the smuts, it is by means of the mycelium 

 that they obtain their nourishment from the plants on which they prey, 

 and it is therefore the vegetative part of the parasite. Hence, although 

 hidden in the interior of the tissues, it is the foundation of all the disease 

 and deformation which afterwards occur. It is not of itself an evident 

 cause of disease in the plant, for it is generally so intimately bound up 

 with the host-plant and grows so regularly with it, that there is usually 

 no external evidence of its presence. It is only when the spores are formed 

 that the fungus is revealed, and then what remains of it is often used up 

 in their production, so that there is no portion of the fungus which is more 

 easily overlooked or less seldom investigated than the mycelium. Commencing 

 in the young seedling, as it generally does, and growing steadily with it until 

 the seed again is formed, it can readily be understood that the fungus filaments 

 are difficult to detect among the tissues. They are most easily seen in the 

 vicinity of the spore-beds, but even in the early stages of their formation 

 they can be detected, as in the neighbourhood of the growing point uf cereal 

 seedli'igs. 



The mycelium is composed of hyaline tubes, which are usually septate, 

 copiously branched and comparatively narrow, being only on an average 

 about 2-5 /( broad (Plate I., a, b). The walls of the hyphae are relatively 

 thick, and sometimes a decided double contour of the membrane is visible 

 (Plate I., c). When treated with iodine and sulphuric acid or chlor-zinc- 

 iodine there is no blue colouration produced, shomng that they are not 

 composed of cellulose. From the transparent nature of the walls and the 

 watery contents of the cells, often filled with vacuoles, it is necessary to use 

 reagents to make the mycelium stand out clearly from the surrounding tissue. 

 If the tissue of the host-plant containing mycelium is kept in a strong solution 

 of potash for 24 hours, the fungus filaments are rendered more distinct in 

 contrast with the clear and transparent tissue surrounding them. The 

 hyphae not only ramify between the cells of the host-plant, but may even 

 penetrate into them and form haustoria, which are seldom spherical, but 

 most frequently like a bunch of grapes. In the cereals, long unbranched 

 hyphse are mostly found in the internodes, Avhile in the nodes the}' are much 

 branched and convoluted. 



Perennial Mycelium. 



I}i the Rusts, the mycelium may cither confine itself to definite spots 

 and become localized, or it may permeate the entire plant, or at least large 

 portions of it. In the Smuts there is a similar arrangement. When the 

 young ovary is infected as in the Loose smut of wheat {Ustilago fritici), the 

 mycelium is restricted at first to that portion of the host-plant, but in most 

 cases the seedling is attacked, and then the mycelium perjneates the entire 

 plant. When the spores are being formed, it is most readily found in their 

 vicinity, but it originally existed throughout the plant, although the earlier 

 formed portions may have disappeared. The mycelium persists more 

 particularly in the nodes, where it remains dormant, but if the host-plaut 

 ir perennial and gives rise to fresh shoots next spring, it awakens into fresh 

 life, and enters the new growth, as in Ustilago perennans, Rostr. 



