296 A TEXTBOOK OF THEORETICAL BOTANY 



The Fungus is heterothallic, and the basidiospores are either of (+) 

 or (-) strain. Ahhough mukiple infections of a leaf by basidiospores 

 may occur, the two strains of myceha can co-exist in the leaf, at least 

 for some time, without fusion, each remaining monoploid. Fusion occurs 



later. 



Four days after infection the mycelia produce pycnidia, mostly on the 

 upper surface of the infected patch or pustule on the leaf. These are small 

 flask-shaped bodies, inside which pycnospores are produced, by abstriction 

 from short hyphae, along with paraphyses (Fig. 288). 



The pycnidium secretes a sugary nectar which oozes out in a drop from 

 the neck on to the leaf surface. The pycnospores, like the mycelia from 

 which they spring, are of ( + ) or ( - ) strains. Fertilization is accomplished 

 by mixing nectar, containing pycnospores of one strain with the exuded drop 

 of nectar over a pycnidium of opposite strain. The older interpretation of 

 the function of the pycnidia was that they were male organs producing male 

 cells, and they w^ere therefore called spermogonia and spermatia respectively. 

 The female organs were supposed to be trichogynes formed by certain hyphae 

 which penetrate outwards to the leaf surface. It is doubtful, however, if 

 these supposed trichogyne hyphae are ever functional, and certainly the 

 usual portal of entry for the " fertilizing " strain of the Fungus is through 

 the existing pycnidia. 



The transfer of nectar containing pycnospores from leaf to leaf is chiefly 

 the work of flies which feed on the nectar. Shortly after a pycnidium has 

 received spores of opposite strain, there must presumably be a fusion of 

 pycnospores in the pycnidium, because hyphae grow from it in which the 

 cells contain two or three nuclei. This is the diploid or sporophytic 

 mycelium, which also ramifies through the mesophyll (Fig. 289). Fertilized 

 pycnidia then dry up and die. 



Meanwhile the primary or monoploid mycelium has built up, on the 

 lower side of the leaf, spherical masses of cells which are the beginning of 

 the aecidia or " cluster cups " (Figs. 290 and 291). If only one strain is 

 present in the leaf these abort and die, but if a diploid mycelium reaches the 

 young aecidium, its cells mingle with the monoploid tissue already formed 

 and produce a tissue in the base of the aecidium from which the aecidiospores 

 are developed (Fig. 292). The sporophytic cells become multinucleate, 

 and from them binucleate spores are cut oflF terminally in chains, thus using 

 up all the extra nuclei but two, which remain in the basal cell of the chain. 

 Each mother cell of an aecidiospore, as it is cut off', divides and the upper half 

 becomes the spore, while the lower half, which is called the intercalary cell 

 soon disorganizes. The wall of the aecidium splits open and the spores are 

 exposed in the cup thus formed. In an aecidium, therefore, the wall tissues 

 are monoploid and the spore-forming cells are diploid. The orange colour 

 of the aecidial cups makes them very conspicuous on the lower surface of the 

 infected leaf. 



The aecidiospores infect Wheat plants or other grasses, germinating on the 

 leaves and sending inwards a primary hypha, generally through a stoma. 



