A NEW SOURCE OF PLANT FOOD 



THE use of leguminous crops for 

 soil improvement," writes Karl 

 F. Kellerman, "has long been 

 recognized by good farmers 

 as desirable. Aside from the general 

 benefits of crop rotation, the actual 

 soil-enrichment is due largely to an 

 accumulation of available nitrogen, and 

 this gain in nitrogen is caused by the 

 presence of minute soil-bacteria which 

 possess the power of growing on the 

 roots of the legumes and then utilizing 

 or 'fixing' the free nitrogen of the air 

 and converting it into food for succeed- 

 ing crops." 



North American farmers have become 

 quite familiar with these nitrogen- 

 gathering leguminous plants, and they 

 play an important part in the cultiva- 

 tion of numerous crops. Cow-peas, 

 velvet-beans, vetches, and other leg- 

 umes are sown between rows of fruit 

 trees and plowed under when they begin 

 to bloom, in order that the nitrogen 

 which they have gathered in the small 

 nodules upon their roots may be utilized 

 by the trees. The use of these green 

 cover-crops lessens the amount of other 

 fertilizer which it is necessary to apply 

 to the orchard. 



Attention has recently been called to 

 certain plants which gather nitrogen, 

 like the legumes, but which store it in 

 small nodules on the leaves instead of 

 on the roots. The one of which a leaf 

 is shown in the accompanying illustra- 

 tion is Psyckotria bacteriophila; it has 

 been shown by Zimmermann and Faber 

 that Pavetta Zimmermanniana and prob- 

 ably other rubiaceous plants have the 

 same habit. These plants, which be- 

 long to the same family as coffee, are, 

 like the latter, tropical in their dis- 

 tribution. 



Faber has shown that these leaf- 

 nodules contain colonies of a non- 

 motile, nitrogen-fixing bacterium which 

 he has named Alyco-bacterium ruhiacea- 

 rum. These bacteria almost invariably 

 inhabit the micropyle of the young 

 seed, and, when the latter germinates, 

 grow through certain stomata of the 

 young leaves and into the intra-cellular 



spaces formed in the leaf-tissues around 

 these stomata. Cavities are formed 

 through the growth of the epidermal 

 cells which later close entirely and make 

 bacterial nodules which are deeply 

 imbedded in the leaf tissues. A single 

 leaf may have several dozen of these 

 symbiotic bacterial nodules. Faber was 

 able, by treating the seeds with hot 

 water and a sublimate solution, to kill 

 the inhabiting myco-bacteria, and, later, 

 to infect part of the seedlings grown 

 from these seeds with pure cultures of 

 the bacterium. The artificially in- 

 fected seedlings grown in soil free 

 from combined nitrogen grew well and 

 remained healthy for four months, 

 whereas those not so infected turned 

 yellowish-white and died in three or 

 four weeks. The plants from unsteril- 

 ized seeds produced leaves bearing 

 many more bacterial nodules than did 

 those from sterilized seeds which were 

 later artificially inoculated. 



In view of the fact that these rubiace- 

 ous plants with nodule-bearing leaves 

 occur in many parts of the tropics, 

 and that in India the value of their 

 leaves has long been recognized, and 

 considering the importance of nitrogen- 

 fixing legumes as soil enrichers, the 

 suggestion of Faber that we may have 

 in these trees and shrubs plants of 

 positive agricultural value deserves the 

 serious consideration of tropical planters. 

 If they can be grown as subsidiary crops 

 beneath plantations of rubber, cacao, 

 coffee, or other important tropical 

 cultures, and their leaves allowed to 

 accumulate upon the ground to serve 

 as a mulch and as nitrogenous fertiHzer, 

 they may have great value. They 

 differ from the leguminous cover-crops 

 in that they are perennial in habit, and 

 will not need to be replanted every 

 year. It might be possible to prune 

 them severely everj^ year and utilize 

 the clippings as fertiHzer. The subject 

 is one which opens up a new field in 

 connection with tropical agriculture, 

 and one which offers remarkable pos- 

 sibilities. 



307 



