408 



BOTANY 



FAPa' II 



sn>i- 



formed by the latter, would together correspond to the asexual generation 

 (sporophyte) of the Ascomycetes. The basidiospores would thus corres}>ond to the 

 ascospores, while the niyceliuui proceeding from the basidiospores and ending in 

 the production.of aecidia would be equivalent to the sexual generation (gametophyte). 

 The tliree forms of spore borne by the sporophyte show, according to Chiustman, 

 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 Ui'edineae being termed autoecious. 

 On the other hand, the sperm ogonia and aecidia 

 may occur on one species of host plant, and the 

 uredospores and 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 (^^). 



An example of an heteroecious Rust-fungus 

 is afforded by Puccinia graminis, the rust of 

 Avheat. It develojjs its uredospores and teleuto- 

 spores on all the green parts of Gramineae, 

 especially of Rye, Wheat, Barley, and Oats. 

 The aecidia and spermogonia of this species are 

 foxind on the leaves of the Barberry (Berberis 

 vu-lgaris). In the spring the hibernating 

 double teleutospores give rise to transversely 

 septate basidia, from which the four basidio- 

 spores are abstricted (Fig. <338, 2). These are 

 scattered by the wind, and if they fall on the 

 leaves of the Barberry they germinate at once. 

 The germ -tube penetrates the cuticle, and 

 sterile cell; a, fertile cells; at an there forms a mycelium which gives rise to 

 the passage of a nucleus from the spermogonia on the upper side of the leaf and 

 adjoining cell is seen ; i?, formation , . ,. ,, ^ -j /i?- o^an mi ■ 



to aecidia on the under side (Ing. 340). This 



form of the fungus is known as Aecidium 

 Bcrberidis. On the rupture of the iieridium 

 the reddish-yellow aecidiospores are conveyed 

 by the wind to the haulms and leaves of 

 grasses, upon which alone they can germinate. 

 The mycelium thus developed produces at first 

 uredospores (Fig. 338, 5). They are unicellular, 

 studded with warty protuberances, and provided with four equatorially disposed 

 germ-pores. In consequence of the reddish-yellow fat globules contained in the 

 protoplasm of the spores, the fructification forms red markings (formerly termed 

 Uredo linearis) on the epidermis of the host plant. The uredospores are capable 

 of germinating at once on the wheat, and thus the rust disease is quickly spread. 

 Towards the end of the summer the same mycelium produces the dark brown, 

 thick-walled teleutospores (Fig. 338, 1), which in this species are always double, 



Fig. 341. — Phragniidium vtolaceum, A, 

 portion of a young aecidium ; st, 



of the first spore-mother-cell sm, 

 from the basal cell a of one of the 

 rows of spores ; C, a further stage in 

 which fromsmi the lirstaecidiospore 

 («) and the intercalary cell (z) have 

 arisen; s»Ho, the second spore-mother- 

 cell ; D, ripe aecidiospore. (After 



Bl.ACKMAN.) 



