14 Reproductive Organs. 



Daiigeard^ found that the young cells of the mycehum or vegetative 

 cells contain two nuclei, although older cells may contain more than two. 

 Then when spores are about to be formed, this formation occurs in special 

 swollen cells, each of which contains two luiclei, as in the purely vegetative cells, 

 a fact also estabUshed by other observers. These nuclei afterwards fuse and 

 form a single nucleus, so that the mature spore is uni-nucleate. This fusion 

 of the nuclei is regarded as a sexual act, equivalent to the conjugation of 

 male and female pro -nuclei. The entire cell with its accumulated reserve 

 material constitutes an oospore, which increases in size and surrounds itself 

 with a thick membrane. Whether this is to be regarded as a sexual act or 

 not depends upon the view we take as to the essence of the process. If it 

 consists in the fusion of two nuclei, representing respectively the male and 

 the female element, derived from more or less speciaUzed cells and forming 

 the single nucleus of a new generation, then the above does not conform 

 to this view, where the blending nuclei are derived from different cells. The 

 fusion of nuclei in the individual spore may serve the same purpose as repro- 

 duction by giving it increased vigour, but it is only where a fertile cell is 

 stimulated to fiu'ther development by the entrance of a nucleus from without 

 that it is here regarded as true sexual reproduction. When the spore of 

 Ustilago, for instance, germinates in water, the single nucleus passes into the 

 germ-tube and divides, then a transverse septum is formed with a nucleus 

 in each division, or the nuclei may divide further and become four, before 

 the septa arise and form the typical four-celled promycelium. When the 

 conidia are formed, each contains normally a single nucleus, which is derived 

 from the division of the nucleus in the parent cell (Plate 1., q and s). 



In Urocystis and Tilletia, where the conidia are formed at the apex of 

 the promycelium, the original nucleus becomes eight by successive bipartition, 

 which pass in order into the apical conidia. 



There is another phase of so-called sexuaUty which has long been observed 

 in Tilletia and other genera, where the conidia unite in pairs. Two conidia 

 become connected by a tube like the transverse bar in a capital H, and 

 resemble very much the conjugating cells of Spirogyra, a fresh- water Alga. 

 Such fusions were naturally suggestive of a sexual union, and De Bary 

 maintained the view that they were analogous to a sexual process. In 

 support of this view he observed, first, the almost invariable occurrence of 

 pairing under the normal conditions of germination, as in water. Second, 

 the union occurred, in the great majority of cases, between two and only 

 two conidia. When an odd number of conidia were produced by the 

 promycelium, they still united in pairs, and the odd one did not fuse with any 

 of the others, although it would have been so easy and so natural to do so. 

 This would seem to show that a change had taken place in the fused pairs 

 which rendered a further imion difficult or impossible. This view of De Bary 

 was generally accepted as being the most probable explanation of the 

 phenomenon, that the conjugation of the conidia was a sexual union. But 

 Brefeld opposes this view, and considers the fusion a purely vegetative act, 

 as occurs in the coalescence of vegetable cells. He regards the essence of a 

 sexual union to He in the fact that the conjugating cells are incapable of 

 further growth by division, but become capable of further development 

 as a result of their union. Applying this test to the conidia, he finds that 

 they are individually capable of unHmited development when placed in 

 nutrient solutions before pairing can begin, and that fusion does not occur 

 under such conditions. He concludes that the fusions are purely vegetative, 

 perhaps resulting from a process of starvation, for so long as adequate 

 nourishment is supplied there is no appearance of conjugation. 



