62 



SCIENTIFIC AGRICULTURE. 



February, 1921. 



were closely related botanieally and when we added 

 fenugreek this still held. Likewise vetch, peas, lentils, 

 broad bean and sweet peas are closely related genera, 



But group III proved the undoing of this theory. 

 The family Leguminosae is divided into three sub- 

 families, Mimosideae, Caesalpiuioideae and Papilionoi- 

 deae. In our group III we have Acacia belonging to 

 the Mimosoideae, Cassia belonging to Caesalpiuioideae, 

 and Vagna, Arachis Lespedeza. Mueuna, Baptisia, Des- 

 modium, and Genista belonging to the Papilionoideae. 

 Obviously the close relation of plaut genera is not 

 con-elated. Of course plant classification is based on 

 morphology and the scheme is more or less arbitrary 

 so that we had no reason to hope for positive results. 



Generally we found that species within a single 

 genera cross-inoculated, but one exception was found 

 to this. The lima bean. (Phaseolus lunatus) was found 

 not to belong in group VI, among the others of the 

 Phaseolus group, but rather in the very mixed Group 

 III. 



One of the early writers. Maze, divided the nodule 

 bacteria into two large groups, claiming that the spe- 

 cial adaptations were due to the reaction of the soil. 

 One group had become adapted to soils of an acid 

 reaction and the other to soils of aii alkaline reaction. 

 But this theory does not ex])lain the existence of eleven 

 groups, and furthermore we have within tliese groups 

 acid-loving and lime-loving plants, thus exploding the 

 theory. Recently some work has been done on opti- 

 mum hydrogen-ion concentration of media for the 

 bacteria, but as yet we have no clear-cut information. 

 The author's opinion is that we are limited to two 

 possible theories. One is that the adaptations are ex- 

 plained by similarity of cell sap, the other that the 

 bacteria contain specific enzymes which are able to 

 dissolve the cell wall*. It may be compared to having 

 a number of Yale locks with keys. 



ATTEMPTS AT BREAKING THE SPECIAL ADAP- 

 TATIONS. — As stated before several investigators re- 

 ported that they had succeeded in bringing about 

 crosses which did not occur naturall.v. One report 

 stated that b.v growing the bacteria on nitrogen-free 

 media for a time the bacteria became nitrogen-hungry, 

 and when then tried on plants other than the origiiuil 

 host the cross was successful. Without going into 

 detail, I will sa.v that all of our efforts in this direc- 

 tion and along similar lines failed absolutely. 



We tried to cause the bacteria to lose their virulence 

 on their own particular host by feeding them organic 

 nitrogen, inorganic nitrogen (as nitrates), and all the 

 various means of abuse we could think of. The autlu)r 

 has grown the bacteria for two years and a half during 

 which time they were in test-tubes containing various 

 kinds of media, but in no case did they lose their viru- 

 lence and in no case did the.v succeed in producing no- 

 dules on plants outside of their original group. Since 

 tins experimental work was reported, there are records 

 of bactei'ia kept for six years luider artificial condi- 

 tions but no change resulted. Tlierefore it seems cer- 

 tain that these adaptations as they occur in nature are 

 permanent and not subject to change: also that the 

 earlier investigations were in error, probably tiimugh 

 faulty technic. 



NON-LEGUMES TP'^t-qiit TO BE ABLE TO USE 

 ATMOSPHERIC NITROGEN.— Certain plants outside 

 of the legume family are known to have nodules on 

 their roots. The list is as follows: 



New Jersey tea (red root), Ceanothus. 



Russian olive, Eloagnus. 

 Alder, Alnus. 

 Black pine, Podocarpus. 

 Sago palm, Cijcas. 

 Sweet gale, Myrica. 



This is not the entire list, but includes the plants 

 with which we worked. All of these plants have nodu- 

 les on the roots much resembling the nodules of leg- 

 uminous plants. Certain investigators, notably Bot- 

 tomly of England, claimed that these nodules contain- 

 ed bacteria identical with those of the nodule bacteria. 

 In fact Bottomley claimed to have inoculated legum- 

 inous plants with bacteria isolated from the nodules 

 of certain of those plants. He also concluded that these 

 plants utilized atmospheric nitrogen as did leguminous 

 plants. 



Our results are as follows: 



1. — The root nodules are different morphologically 

 from the nodules of leguminous plants. 



2. — The root nodules are not caused by the bacteria 

 which cause the nodules on leguminous plants. Nu- 

 merous efforts to isolate bacteria failed, and the 

 methods used were identical with those which were 

 successful with legume root nodules. 



3. — The evidence points to a fungus as being the 

 cause of the nodules on these non-legumes. 



4. — These non-legumes are not able to utilize atmos- 

 pheric nitrogen as demonstrated b.v trial in pots of 

 sand, the seedlings being inoculated by appl.ving 

 crushed nodules, and being compelled to depend upon 

 the air for their nitrogen supply. 



Therefore we were forced to the conclusion that 

 symbiosis is confined to the legume family, and to our 

 knowledge does not exist outside of this family. 



Our work was interrupted by the death of Doctor 

 Burrill in 1916, but we had learned much concerning 

 the subject. The project seemed less hopeful at the 

 end that at the beginning. However, negative results 

 are not without value. 



In conclusion, several of the important facts regard- 

 ing the nodule bacteria, ma.v be given, as follows : 



First. — Leguminous plants when inoculated by the 

 proper bacteria, as evidenced by the presence of no- 

 dules on the roots, are able to utilize nitrogen of the 

 air. 



Second. — This .s.vnd)iotic relation exists only in the 

 legume family, and to the writer's knowledge does not 

 occur in any non-leguminous plants, either naturall.v 

 or otherwise. Within the legume famil.v no species 

 exists which does not possess this relation providing 

 the proper bacteria are present. 



Third. — Leguminous plants ma.v be divided into 

 groups according as they can be cross-inoculated b.v 

 certain bacteria. These bacteria have become adapted . 

 to certain plants or groups of plants, and these adapta- 

 tions which occur naturally are persistent and could 

 not be broken down by repeated trial. 



Fourth: — These special adaptations cannot be ex- 

 plained on the basis of close botanical relationship or 

 soil reaction. Two theories are possible, one basing the 

 adaptations on similarity of the root-.sap of the plant.s, 

 and the other presuming the existence of specific en- 

 zymes produced by the bacteria capable of dissolving 

 the cell wall of the root hairs or root epidermis. 



Fifth. — The cultivation of the bacteria in various 

 kinds of media through long periods of time did not 

 destroy their virulence. 



