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UREDINEAE 



Nor arc these plants less interesting from the purely Scientific point ot \ lew. 

 The majority of the rusts have developed the curious (acuity ot* living 

 partly on one host-plant and partly on another. Such a fungus is said to 

 be hetevoecious. No case ot this kind is yet known in South Africa, 

 but there can he little doubt that some do occur here as well. As the two 

 stages ot these fungi are very different, they were tor a long time thought 

 to represent wholly different diseases. 



Taking one ot the commonest ot the wheat rusts, Pucciltia gramixtS, .is 

 a type ot the Uredineae (Fig. 25), we rind that the disease caused by 

 it on the wheat plant has two distinct phases. At first, while the host 

 plant is still young and green, the disease appears as orange-coloured streaks 

 and patches, called sori, on the stem and leases ; later, when the plant 

 becomes old and straw-coloured, the sori rapidly assume a dark rust colour 



Fig. 2]\ Puccinia graminis. Accklium on leaf of barberry [Berberh vulgaris), 

 rp, epidermis of lower side of leaf; ///, intercellular mycelium ; />, pel idium ; 



.(, spores. 142/1. (From Strasburger, Textbook) 



(big. 25, 1 and 2). The fungus has no other form in South Africa, nor 

 does it occur, as far as is known, on any other unrelated host-plant ; but in 

 Europe the remaining phases are found on the leaves ot barberry plants. On 

 the under surface of these leaves the fungus appears as little cup-shaped 

 structures, quite visible to the naked eye, while on the upper surface minute 

 brown dots, only with difficulty visible to the eye, are to be seen. Turning 

 to the microscopic structure of the fungus in these different stages, we find 

 that the orange-coloured sori on the wheat plant contain large numbers of thin- 

 walled spores, called tiredospoves (Fig. 25, 5, on right), on short stalks. 

 These are capable of germinating at once on the same or other cereal, and in 

 a week a single spore may develop to form a mycelium in the tissues, and 



