EU-MYCETES. (b) BASIDIOMYCETES 449 



sterile intercalary cells (z) (Fig. 384). Each chain of spores appears to be 

 initiated in this way. A similar process has been seen in P. speciosum, and 

 other Uredineae. This discovery made it possible to relate the life-history 

 of the Uredineae to that of other plants. The stage with paired nuclei is 

 held as the correlative of the diploid sporophyte, and that with a single 

 nucleus in each cell as the haploid gametophyte. The events may ttien 

 be summarised as follows : 



A. Sporophyte on Grass (paired nuclei). 



i. Mycelium. 



ii. Uredo-spore (repeating the mycelium), 



iii. Teleuto-spore (winter's rest), 



iv. Nuclear fusion in germinating Teleuto-spore. 



v, Basidium ( = promycelium), equivalent of a spore-tetrad, with 



reduction. 



vi. Carpospore ( = sporidium). 



vii. Infection of Barberry plant. 



B. Gametophyte on Barberry (uni-nucleate). 



viii. Mycelium. 



ix. Spermogonium (non-functional). 



x. Apogamous nuclear association. 



xi. Aecidium-spore (bi-nucleate). 

 xii. Infection of Grass-leaf. 



There are many of the Uredineae that do not show so elaborate a series 

 of stages in the life-cycle as the Rust of Wheat. The uredospores may be 

 omitted, or the aecidia ; *or some may have neither aecidia nor uredo-spores. 

 The spermogonia may also be omitted : so that in these last the life consists 

 of a repetition of teleuto-spores with subsequent germination. 



The Rust disease is difficult to check, and its distribution is world- 

 wide. One obvious measure would appear to be to remove the 

 alternative host, the wild Barberry. This has been done in wheat- 

 growing districts in England, but definite consequences from it are 

 uncertain. It is significant that Wheat-Rusts abound in South 

 Africa, Australia, and parts of India, where no species of Berberis 

 are indigenous. The most effective remedy is to plant only the seeds 

 of varieties of wheat that are known to be immune to the disease, by 

 resisting the infection. Progress has already been made in the 

 production and circulation of such immune varieties. But owing 

 to the minute specialisation characteristic of many Rusts a variety 

 may be immune in one country and susceptible to the same Rust 

 in a different climate : so delicate is the balance which exists between 

 the attacking and resistant powers of two organisms. (Compare 

 Chapter XI, on Irregular Nutrition.) 



B.B. 2F 



