Vegetative Organs. 5 



Perennial Mycelium. In contrast to the localized mycelium, there may- 

 be a mycelium with unlimited growth which does not confine itself to par- 

 ticular spots, but may permeate entire shoots, or evem the whole plant. 

 This is known as a perennial mycelium, and wherever it occurs the fungus 

 may reappear on the same plant year after year without the necessity for 

 reinfection by means of spores. 



This vegetative reproduction through a perennial mycelium is not always 

 easy to prove, but its importance cannot be overrated, for hidden in the 

 tissues of the plant it cannot be reached by the ordinary means for con- 

 trolling the growth and spread of fungi, but involves the destruction of 

 the plant, or at least of those parts which harbour it. As Australian ex- 

 amples, we may note Uromyces trifolii, which attacks the white clover 

 (Trifolium repens) and Phragmidium subcorticium or rose-rust, in which 

 the mycelium of the aecidial stage penetrates all the tissues, an3 in each 

 succeeding year forms a new layer beneath the old. 



Uromycladium notabile and U. tepperianum occurring on species of 

 Acacia are further examples, for the mycelium gives rise to large galls, 

 which persist from year to year and produce spores. 



Even although the host-plant is an annual, and dies down every year, 

 it is still possible for the mycelium to be perennial, for it may be carried 

 over winter in the seed, as in the case of Uromyces euphorbiae, according to 

 Carleton 3 . 



Witches'-brooms. It is not unusual among the forest trees and shrubs of 

 Europe to find shoots very much deformed and distorted, and looking 

 at a distance like large birds' nests or brooms, and to these the popular 

 name of witches '-brooms has been given. These peculiar and diseased 

 conditions were difficult to account for, and so the idea may have originated 

 with superstitious people that the trees were bewitched, in order that the 

 witches might be provided with brooms for their midnight rides, hence 

 the name. 



But the true cause is seen when the matter is investigated in the light 

 of our present knowledge, and parasitic fungi a/re often found to be respon- 

 sible for the strange transformations of the normal shoots into the dense 

 twiggy, irregular tufts met with. This may also be produced by other 

 means, such as gall-mites, but a very striking case and the first recorded 

 instance in Australia is thalt of the rust-fungus, Cronartium jacksoniae, 

 which deforms the shoots of various leguminous plants as shown in 

 Pis. XXXVII., XXXVIII. Uromycladium tepperianuni also produces 

 this peculiar appearance on Acacias as shown in PL XLII. 



The perennial mycelium in the shoots stimulates a number of buds to 

 abnormal growth, quite different from the ordinary, and the result is seen 

 in the numerous densely crowded and considerably altered shoots as com- 

 pared with the normal. They are also thickly studded with the ruddy 

 brown columns of teleutospores, in the case of Cronartium, which stand out 

 like so many curved or straight, stiff bristles, towards the ends of the 

 shoots, which are gradually being destroyed. Next year the mycelium will 

 grow into the young shoots and produce the same result. 



Formation of Galls. It is well known that the mycelium of fungi exer- 

 cises a stimulating effect upon growth, and not only causes the cells to grow 

 larger and divide more frequently than usual, and the chlorophyll to dis- 

 appear, but it may alter the character of the tissues. When an insect pierces 

 the young and living tissue of a plant with its proboscis or ovipositor. 

 it often causes the cells immediately surrounding it to grow and divide 

 more rapidly than elsewhere, so that a swelling of the tissue occurs, which 

 is known as a gall. So among the rusts there are instances where the stimu- 

 lation of growth occurs in a marked degree, and if a vegetable gall be 



