McrobioJ :'.i 



ThRre are tvjo groups of substances involved, namely, (a) cellular 

 debris, including sloughed-off root hairs, root caps, and cortical 

 and epidermal cells, and (b) excreted materials, as for example amino 

 nitrogen compounds. Infonnation concerning the amounts of organic 

 materials released in the rhizosphere is limited. 



Lyon and Wilson (9) measured the release of organic matter from roots 

 of plants grown in sterile nutrient solutions and found it to be of 

 the order of 3 percent of the total weight of the plants at matiu-ity. 

 'iJhether or not this value represents a good estimate of the loss to 

 the rhizosphere by plants growing \inder natural soil conditions, I am 

 not prepared to answer. 



Another problem concerning the natiiral rhizosphere is the satisfactory 

 differentiation between excretory products as such and the decompo- 

 sition products arising either from cell autolysis or as a consequence 

 of microbial attack upon cellular debris. Roviar (10) has presented 

 some meas-urements of the amounts of material given off by roots of pea 

 and oat plants during early stages of growth in quartz sand. For these 

 plants, after both 10 and 21 days of growth, the soluble organic mate- 

 rial formed the bulk of the material coming from the roots. He assujned 

 that the bulk of the soluble exudate was true excretion, not produced 

 by lysis of cells, because of the observation that the amount of cellu- 

 lar debris was doubled betvreen 10 and 21 days, wliile the amount of 

 soluble material only increased by half. At 10 days the dry weight of 

 material excreted was three to fourfold the dry weight of root debris; 

 at three weeks, the amount of exudate was little more than double that 

 of the sloughed-off cells. He concluded that in older plants true root 

 excretions probablj'- are not as important in supporting the rhizosphere 

 population as is the cast-off cellular material. 



It is becoming apparent that the compounds that can be excreted from 

 roots are many and varied. Not many years ago, we were content to 

 think primarily in terms of carbohydrate materials of varying complex- 

 ity and of amino nitrogen compounds. There was even some disagreement 

 about the excretion of amino acids. Recently Winter (11) has stated 

 that in addition to sugars and amino acids, such compounds as the 

 follot'jing come from plant roots: phosphatides, alkaloids, tannins, 

 thisjiiin, boitin, mesoinositol, and para-amino benzoic acid. Curently, 

 m.any specific additions are being made to this list, as chromatographic 

 techniques permit the isolation and identification of hitherto unknoim 

 chemical compounds. A concurrent development, stemming largely from 

 interest in systemic pesticides for plants, is the recognition that 

 copplex organic molecules can be ti'ansportod, excreted, and taken up 

 by plants. 



Having noted some of the nutrient materials that arise from roots, we 

 may novr consider briefly a question of origin of the rliizosohere micro- 

 flora. There have been soma recent sugr.ostions that certain components 

 of rhizosphere population m-lr.c: from specific sources. Gyllenberg (12 \ 



