848 KEPORT— 1897. 



No treatment of this subject would he complete without reference to those 

 ohscure cases of symbiosis — as we must regard them — between certain algiB which 

 occur in the cavities of the leaves of Azolla and in Gunnera, and those found in 

 the intercellular spaces of cycad-roots. When we know more of the physiology 

 of these blue-green algse, it may be possible to explain these puzzles, but at present 

 they are mysterious curiosities. 



A class of pseudo-symbiotic organisms is being more and more brought into the 

 foreground where the combined action of two symbionts results in death or 

 injury to a third plant, whereas each symbiont alone is harmless, or compara- 

 tively so. 



Some time ago Vuillemin showed that a disease in olives results from the inva- 

 sion of a bacillus (2?. olecc), which, however, can only obtain its way in the tissues 

 through the passages driven by the hyphse of a fungus ( Chcetophomd). The result- 

 ing injury is a sort of burr. Vuillemin has this year observed the same bacillus 

 and fungus in the canker burrs of the ash, and so confirms Noack's statement to 

 the same effect. 



Among many similar cases, well worth further attention, the invasion of 

 potato-tubers by bacteria, which make their way down the decaying hyphae of 

 pioneer fungi, may be noted. I have also seen tomatoes infected by these means, 

 and have facts showing that many bacteria which quicken the rotting of wood are 

 thus led into the tissues by fungi. 



Probably no subject in the whole domain of cryptogamic botany has wider 

 bearings on agricultural science than the study of the flora and changes on and in 

 manure and soil. 



As vegetable physiology and agricultural science progressed, it became more 

 and more of primary importance that we should learn what manure is composed 

 •of, what changes it undergoes in the soil, and what the roots of plants do with it. 

 <Jhemistry did much to solve some of the earlier problems, but it soon became 

 evident that it only raised new questions which it could not solve ; and it was not 

 till the sequence of changes induced by the successive growths of Mucor, Pilobohts, 

 Coprinus, Ascobolus, and other moulds and fungi of various sorts, followed by 

 bacteria and yeasts, began to be understood, that anything approaching a coherent 

 account of the complex phenomena going on in soil or in a manure-heap could be 

 attempted. Not that all the difficulties have been solved even now-, but we are at 

 least able to trace some very important chains of occurrences which throw light 

 on many hitherto obscure matters going on in the field. 



Since Pasteur in 1862, and Van Tieghem in 1864, showed that certain bac- 

 teria are concerned in converting urea to ammonium carbonate, much has been 

 iearnt, and we now know from the investigations of Miquel, Jaksch, Leube, and 

 ■others that numerous urea-bacteria exist; and Miquel, in 1890, isolated an ex- 

 tremely unstable enzyme — urase — which converts sterile urea to ammonium 

 ■carbonate very rapidly, a discovery of considerable interest, as it was one of 

 the first examples of this class of bodies to be examined ; and when we reflect 

 on the enormous quantities of urea which have to be destroyed daily, and that 

 fresh urine is in effect a poison to the roots of higher plants, some idea of the 

 importance of these urea-bacteria is obtained. The necessity for preventing the 

 losses of this volatile ammonia by fixing it in the soil and presenting it to the 

 action of the nitrifying organisms is also obvious. 



Winogradsky's classical isolation and cultivation of bacteria which take up 

 these ammonia compounds and oxidise them to nitrous and to nitric acids in the 

 soil, may be quoted as further instances of the bearing of bacteriological work on 

 this department of science, as explaining not only the origin of nitre-beds and 

 deposits, but also the way the ammonia compounds fixed by the soil in the neigh- 

 bourhood of the root-hairs are nitrified and so rendered directly available to 

 plants. 



The theoretical explanation of many questions connected with the washing 

 out of nitrates from fallows, the advantages of autumn and winter sowing, and 

 processes occurring in the upper soil as contrasted with subsoil, has been rendered 

 much easier by these researches ; moreover, as is now well known, they brought 



