DIV. I 



THALLOPHYTA 



455 



cells which form the spores arise. On the other hand, V. Carlo and the majority 

 of the Ustilagineae attain the binucleate condition by a process of fusion between 

 pro-mycelial cells, sporidia, or the cells of the mycelium arising from these (Fig. 

 402). This also holds for Tilletia in which the sporidia before they are shed are 

 united in pairs, the nucleus from one sporidium passing into the other. The 

 hyphal cells and secondary sporidia and the cells of 

 the parasitic mycelium are therefore binucleate. 



In the various Ustilagineae the haploid and diploid 

 phases do not exactly correspond. 



Order 2. Uredineae (Rust Fungi) ( 77 > 78 ) 



The mycelium of the Uredineae lives parasitically 

 in the intercellular spaces of the tissues of the higher 

 plants, especially in the leaves, and gives rise to the 

 widely-spread diseases known as Rusts. Their more 

 varied spore-formation is a distinguishing feature as 

 contrasted with the Ustilagineae. 



As in the latter order, the basidia are not produced 

 directly on the mycelium but on the germination of a 

 special type of spore, TELEUTOSPOKES or winter spores, 

 which are characteristic of all Uredineae. The teleuto- 

 spores arise in small clusters beneath the epidermis 

 of the diseased leaf from the ends of hyphae ; fre- 

 quently two or more form a short chain. They are 

 thick -walled resting -spores and persist through the 

 winter (Fig. 403, 1, 5 t). The group of spores usually 

 bursts through the epidermis. At first the spores, like 

 the cells of the mycelium which bears them, have two 

 nuclei, but the nuclei fuse before the spore is ripe. 



In the germination of the teleutospore a BASIDIUM 

 (promycelium) grows from each cell (Fig. 403, 2) ; it 



becomes divided by transverse septa into a row of four cells from each of which 

 a sterigma bearing a single uninucleated BASIDIOSPORE (sporidium) is produced. 

 The sporidia are dispersed by the wind and germinate in the spring on the leaves 

 of host plants (which may be of the same or different species from the one on 

 which the teleutospores were produced), giving rise to an intercellular mycelium, 

 all the cells of which are uninucleate. From this mycelium organs of two kinds 

 arise, spermogonia on the upper surface of the leaf and aecidia on the lower 

 surface. 



The SPERMOGONIA (Fig. 404) are flask-shaped structures, the base of which is 

 covered with the projecting ends of hyphae ; from these are abstricted spermatia, 

 each of which has a single nucleus. Morphologically they are completely com- 

 parable to the similarly-named male sexual organs of some Ascomycetes ; among 

 the Basidiomycetes they persist only in the Uredineae, and even in them are no longer 

 functional and may be completely wanting. In nutrient solutions the spermatia 

 may put out short germ-tubes, but are not capable of infecting the host plant. 



The AECIDIA (Fig. 405) are cup-shaped fructifications, which when young are 

 closed, but later open ; from the ends of the hyphae numerous closely-associated 

 chains of spores are abstricted. As a rule the enveloping layer or peridium of the 

 aecidium is formed of thick-walled cells corresponding to the sterilised peripheral 

 rows of spores. In Phraguiidium violaceum, which occurs on the leaves of the Black- 



Fic. 402. Ustilago Carbo. A, 

 Conjugating sporidia. B, The 

 two uppermost cells of a 

 promycelium fusing to give 

 rise to a binucleate cell. C, 

 Conjugation between two 

 promycelia. (x 1000. After 

 RAWITSCHER.) 



