442 



BOTANY OF THE LIVING PLANT 



The basidium resembles the ascus in containing two nuclei when young. 

 As in the ascus these first fuse with one another, and then follows division, 

 first into two then into four, which is accompanied by nuclear reduction. 

 In the more primitive types (Uredineae, Tremellini), the basidium divides 

 after the fusion : in the Rusts it appears as a short mycelium (promycelium) 

 from which four cells are partitioned off ; each of these puts out a short 

 beak, or slerigma, and bears a carpospore (Fig. below, p. 446). In the more 

 advanced types, such as the Toad-stools and Puff-balls, the basidium does 

 not divide into distinct cells ; but sterigmata are formed, and each produces 

 a basidiospore or carpospore, which behaves in the same way as in the 

 Uredineae. The difference is that the basidium remains undivided. (Fig. 374.) 



UREDINEAE. RUST-FUNGI. 



The Rust-Fungi are very prevalent parasites on the shoots of many 

 plants, and as their mycelium lives in the intercellular spaces of 



the infected areas, and draws 

 nourishment from the cells, the 

 host-plant suffers, though it 

 struggles on without being killed. 

 If the host be perennial, the fall 

 of the leaf, or dying down of the 

 aerial shoot, rids it temporarily 

 of its enemy, and the parasite 

 has to make a new infection in 

 the succeeding season. It is only 

 occasionally, as in Gymnospor- 

 angium on the Juniper, that the 

 fungus perennates in the host. 

 A very large number of Rust- 

 Fungi are known, and they show 

 a high degree of specialisation in 

 their parasitism, being mostly 

 restricted to certain genera, or 

 even species of host ; while in 

 not a few cases their elaborate 

 life-cycle is completed by stages 

 of growth upon two distinct and 

 successive hosts. This is des- 

 cribed as Heteroecism. The 

 name Rust is derived from the 

 fact that the spores produced 

 by. them in summer are of the 



PIG. 375. 



Upper portion of a stalk of Wheat, with groups of 

 Uredo-spores (u) on the leaves, and of Teleuto-spores 

 (/>) on the fast-ripening leaf-sheath and straw. (After 

 Marshall Ward.) 





