96 BACTERIA IN RELATION TO PLANT DISEASES. 



these varieties are commercially less desirable than some of the sensitive ones. Frequently 

 they are quite inferior except in hardihood. Systematic cross-breeding experiments are 

 here in order. These should be taken up as a part of the regular work of Departments of 

 Agriculture and Experiment Stations and carried on without interruption for a long series 

 of years long enough to insure good hybrids of fixed quality. In some cases systematic 

 selection within the limits of susceptible varieties might lead to useful results. 



A priori, it would seem that something might be done to increase the resistance of 

 plants to disease by judicious feeding. This subject is also in its infancy. Laurent's experi- 

 ments point in this direction, but the question is how much can be accepted as really 

 established by him. Jenssen got contradictory results. Also in the writer's experiments, 

 which were made on a large scale two years running, using the green shoots of potatoes, 

 Bacillus coli was not induced to take on a parasitic life by any variation in feeding. Similar 

 negative results were obtained with Bacillus aroideae and Bacillus carotovorus. Moreover, 

 even with a distinct parasite like Bad. solanacearum, no conclusive or striking results were 

 obtained. Laurent, however, did not claim that decay could be produced by inoculations 

 from cultures of B. coli. The soil in my experiments was Potomac river loam. The ferti- 

 lizers were lime, sodium nitrate, sulphate of potash, muriate of potash and superphosphate 

 (boneblack) . The inoculations were into growing shoots using young agar cultures diffused 

 in sterile water and injected hypodermically. The varieties were Early Rose and Burbank. 

 Three strains of Bad. solanacearum were used, i. e., from Maryland, Georgia, and Porto 

 Rico. Each hill of potatoes in a particular plot received at planting time some one of the 

 following doses: 



Nitrate of potash 8 grams or 16 grams 



Sulphate of potash 8 grams 16 grams. 



Muriate of potash 8 grams 16 grams. 



Lime 25 grams 75 grams. 



Boneblack 8 grams 16 grams. 



This work should be repeated on other soils, applied to all sorts of diseases and extended 

 in various ways. 



Here is a great field for exact experiment. Truckmen, orchardists, and ordinary farmers 

 often stumble on facts of great value, but usually no accurate records are made and conse- 

 quently the scientific man can not make use of their findings for the benefit of others. My 

 experience has been that it is difficult to persuade a farmer to treat part of a field in some 

 particular way and hold the remainder as a check. He usually wishes to treat it all or not 

 at all. Experienced gardeners have beyond doubt the most exact knowledge on this subject, 

 but theirs also is mostly a rule of thumb which they can not readily impart to others. 



SYMBIOSIS. 



Symbiosis according to its Greek derivation is simply life with another organism. In 

 biology it has come, however, to have a restricted meaning. We may have life with another 

 in which the two organisms are indifferent to each other. This, of course, is not symbiosis. 

 Parasitism also is life with another, but the essential idea involved in the concept is antagon- 

 ism and not what we mean by symbiosis. The latter is a helpful life with another, a friendly 

 give and take. The word was invented before we knew much about the conditions to 

 which it was applied. Very few examples of true symbiosis are known, or at least thoroughly 

 worked out. Perhaps there are none in which the giving and taking are equal, and no harm 

 done to either organism. It seems to grade into parasitism. The lichen thallus is an old 

 and familiar example, but here the alga gets the worst of the arrangement, being dwarfed 

 by the fungus, held prisoner and given nothing probably which it could not obtain by itself, 

 unless perhaps in the case of the rock-lichens. Bacterial symbiosis is the only sort falling 



