1060 REPORT— 1898. 



iiost-plant itself, as by its dependence upon the seasons, changes in it necessarily go 

 forward, which will serve as direct occasion for the growth of the different spore 

 formations of the parasite. 



In Uromyces Polygoni, for instance, the a3cidia appear only on the young plant, 

 the uredospores and teleutospores on the older. In the hetersecismal Uredineae, 

 the special conditions for the individual fruit formations are much more strongly 

 distinguished still, so that other host-plants are necessary to bring the Fungus to 

 the formation of lecidiospores or teleutospores. The time will probably come 

 ■when these conditions may be more accurately recognised, and the Uredinese be 

 cultivated on artificial substrata. Then it will appear whether these parasites do 

 not behave just like the other Fungi, and cannot also be compelled to produce the 

 diflTerent fruit formations upon the same mycelium. A great obstacle to the culti- 

 vation of the Uredineae lies in our ignorance of the chemical composition of the 

 host-plants. We are quite ignorant of the substances characteristic for the species, 

 which, besides the usual food-stutfs, sugar, proteid, &c., are at any rate of decided 

 importance for the development of the parasite. 



In all the cases now mentioned we have to do with the alternation of several 

 generations, each of which is characterised by special propagation. In the uni- 

 cellular Thallophytes the non-sexual propagation coincides with the vegetative 

 division. The propagation of the Desmidiacepe and Diatomese by division corre- 

 sponds to the propagation of Chlamydomonas by means of motile cells. In all of 

 them the sexual process ensues after a series of divisions. Naegeli includes these 

 processes under his conception of alternation of generations, and even extends it to 

 the Bacteria in which, after a series of generations by division, the cycle is closed 

 by the formation of endospores. But if the term alternation of generations be 

 limited to organisms with dimorphic propagation an alternation of shoots might 

 be spoken of in this case. 



Under all circumstances, whatever name the thing may bear, we must ask our- 

 selves the same question as regards these phenomena as in the dimorphic Thallo- 

 phytes ; we must inquire whether a more or less definite number of cell-generations 

 must be passed through before the fruit generation can follow. In the bacillus of 

 anthrax {Bacillus anthracis), Buchner (1890) has already proved that it can be 

 propagated as long as you please by division, and that at any moment the forma- 

 tion of spores can be induced by direct influence of the outer world. Schreiber 

 (1896), who has closely investigated the conditions of spore-formation in several 

 Bacteria, has been able to prove still more definitely that the spore-formation 

 always begins as a direct consequence only of external conditions. Starting from 

 the germinating spore, it was possible, after the third division, to induce spore 

 formation again. In Chlamydomonas, I was able to prove with certainty that the 

 cells, through innumerable generations, propagated in an exclusively vegetative 

 manner, but that at any time the formation of sexual swarm spores can be attained 

 ■with ease and certainty. Most probably the same holds for the Desmidiacese, in 

 which certain species may be propagated for many weeks together by division, but 

 from the first were capable of sexual propagation when exposed to the conditions 

 characteristic for it. 



On the other hand, the Diatomeae seem to have a necessary alternation of 

 generations, just as, according to the investigations of Maupas, the Ciliatfe among 

 the Infusoria. According to the theory founded by Pfitzer, the cells of the 

 Diatomere, whose silicitied cell-wall consists of two parts fitted one within the 

 other, do not grow in the direction which is usually styled longitudinal. The 

 consequence of this is that on each division one of the daughter-cells maintains 

 the length of the mother-cell, the other will necessarily be smaller by the thick- 

 ness of the membrane. Thus bv continued divisions the cells become smaller and 

 smaller till, on reaching a certain minimum size, the process of auxospore forma- 

 tion appears, by which the original maximum length is again attained. This 

 generally acknowledged theory has been supported by the investigations of Miguel. 

 He cultivated a number of diatoms in artificial nutritive media, and noted in the 

 successive generations a gradual lessening of the cells, till finally the formation of 

 auxospores followed very freely. Thus, according to the statements of Pfitzer, 



