CHAPTER V. COMPARATIVE REVIEW. USTILAGINEAE. l8l 



conjugation of the segments of the promycelium, or primary sporidia, and that in 

 Urocystis Violae, whose development was also fully observed by Kiihn, conjugation 

 must be supposed to take place from Prillieux' figures, though it is not expressly de- 

 scribed. On the other hand there are species, which, according to our present observa- 

 tions, do not conjugate. But we have no complete observations of their development ; 

 only from the data which we possess, and the positive result of Kiihn's extended re- 

 searches into the infection of host-plants with Urocystis Maidis, it must be supposed 

 that the endophytic sporogenous mycelium is in this case developed directly from the 

 resting-spore or from interpolated gonidia which do not conjugate. Accordingly the 

 course of development, apart from certain special formations and complications which 

 vary in the different species, would be the same in its main features in the entire 

 group of the Ustilagineae up to the difference which lies in the presence or absence 

 of conjugation. 



What importance is to be assigned to this difference is open to discussion. The 

 pairing I have formerly called conjugation and do so still, thus giving expression to 

 the view, that it may or ought to be considered as analogous with a sexual process. 

 Brefeld does not admit this and explains it to be an analogous phenomenon to the 

 coalescence of vegetable cells, especially young germ-tubes (see on page 2). In 

 such cases as these no decisive argument for or against is to be found in the phe- 

 nomenon in itself, but we must look about for indirect grounds of probability. Mine 

 are as follows, if I confine myself to cases that are thoroughly known. First, the 

 almost invariable occurrence of pairing under the normal conditions of germination ; 

 the conditions, I mean, to which the species is actually adapted in nature. The 

 conditions are those for germination in water in the case of Tilletia, the species of 

 Entyloma, Urocystis, and Tuburcinia Trientalis, as has been already shown; and here 

 pairing takes place so promptly that it is not easy to keep primary sporidia from 

 uniting, if we disregard some special exceptional cases mentioned above. But these 

 cases are of the kind which prove the rule. Secondly, the great predominance of 

 union in pairs. The sporidia which are placed close to one another in whorls in 

 Tilletia, Entyloma, Urocystis, and other genera unite, almost without exception, in 

 pairs only, and where there is an odd sporidium it usually does not conjugate, 

 though its union with some pair would be easy, one might almost say would be very 

 natural. The segments of the promycelium of Ustilago Carbo conjugate when they 

 are immediate neighbours in a mode similar to the ' clamp-connection ' of page 2 ; 

 under different local relationships pairing is effected in a different way, as by loop- 

 unions between two segments of a promycelium which are separated by a pair which 

 have already united. These facts show that a change usually takes place in a pair 

 after conjugation which renders a second union difficult or impossible, while it 

 introduces the further development. All these are phenomena which find their 

 analogy, as far as we know at present, only in sexual processes, or, as it may be 

 briefly expressed, in sexual processes of conjugation, and must be interpreted by 

 them, till we obtain further knowledge. The case is different in the coalescence of 

 germ-tubes, which are the nearest comparable. A glance at Fig. i shows that in 

 that case coalescence may take place between any number of spores or may even be 

 omitted, while the further development is the same in kind, but shows different 

 degrees of strength according to the number of the germs which have coalesced. It 



