458 



BOTANY 



PART II 



(Fig. 403, 5, 6). They have two nuclei like all the cells of the mycelium developed 

 from the aecidiospore. They serve commonly to ensure the spread of the fungus in 

 the summer. Later, either in the same or in distinct sori, the teleutospores are 

 formed and in these the fusion of the two nuclei to a single one takes place ; such 

 a fusion as a rule is found to take place in the young basidium. 



The two types of cell fusion in the formation of the aecidium are also known in 



other Uredineae, and must be regarded as replac- 

 ing a formerly existing method of fertilisation. 

 If we attempt to derive the Uredineae from the 

 Ascomycetes the spermatia must be regarded 

 as now functionless male cells, and the so-called 

 fertile cells in the young aecidium as corre- 

 sponding to carpogonia. Extending the com- 

 parison further the mycelium proceeding from 

 the aecidiospore in the Uredineae and the uredo- 

 spores and teleutospores borne on it, together 

 with the basidia, formed by the latter, would 

 together correspond to the diploid asexual 

 generation (sporophyte) of the Ascomycetes. 

 The basidiospores would thus correspond to the 

 ascospores, while the mycelium proceeding from 

 the basidiospores and ending in the production 

 of aecidia would be equivalent to the haploid 

 sexual generation (gametophyte). The agree- 

 ment between Ascomycetes and Uredineae is 

 also shown in the behaviour of the sexual nuclei 

 which only become associated in pairs to fuse 

 later in the yoimg ascus or the young basidium. 

 The three forms of spore borne by the sporo- 

 phyte show, according to CHUISTMAN, a certain 

 agreement in their development from the " basal 

 cell " from which they arise ; they may thus be 

 regarded as morphologically equivalent. 



The life-history of the Rust Fungi is thus a 

 complicated one. The several forms of spore may 

 appear in the course of the year on the one 

 host, such Uredineae being termed autoecious. 

 On the other hand, the spermogonia and 

 aecidia may occur on one species of host plant, 

 and the uredosporesand teleutospores on another, 

 often unrelated, plant. In these heteroecious 

 species there is thus an alternation of host 

 plants. There are also pleophagous heteroecious Uredineae in which the aecidia 

 or the uredo- and teleuto-spores appear on a number of distinct host plants ( 79 ). 



An example of an heteroecious Rust Fungus is afforded by Puccinia graminis, 

 the Rust of Wheat. It develops its uredospores and teleutospores on all the green 

 parts of Gramineae, especially of Rye, Wheat, Barley, and Oats. The aecidia 

 and spermogonia of this species are found on the leaves of the Barberry (Berberis 

 vulgaris). In the spring the hibernating double teleutospores give rise to trans- 

 versely septate basidia, from which the four basidiospores are abstricted (Fig. 

 403, 2). These are scattered by the wind, and if they fall on the leaves of the 



PIG. IQG.Phragmidium violaceum. A, 

 Portion of a young aecidium ; st, 

 sterile cell ; a, fertile cells ; at a% 

 the passage of a nucleus from the 

 adjoining cell is seen. B, Formation 

 of the first spore-mother-cell sw-i, 

 from the basal cell a of one of the 

 rows of spores. C, A further stage in 

 which from sm^ the first aecidiospore 

 (a) and the intercalary cell (2) have 

 arisen; sm^, the second spore-mother- 

 cell. D, Ripe aecidiospore. (After 

 BLACKMAN.) 



