PLANT-TISSUE CULTURES — WEINTRAUB 359 



tomato roots in culture for more than 7 years. Since the roots were 

 customarily subcultured at weekly intervals, this period represents 

 approximately 350 transfers. Under the culture conditions em- 

 ployed, the excised roots elongate at a rate substantially equal to that 

 of normal intact roots, averaging about 5 millimeters per day and 

 producing one or two new branch roots daily. At this rate, if one 

 started a culture with a single 10-millimeter-long root tip which at 

 the end of one week was divided into seven pieces, each including 

 a growing point capable of further development, and continued this 

 procedure each week, maintaining all the subcultures so derived, 

 there would be produced in 4 months a total length of root equal 

 to the distance from the earth to the sun. In 7 years the length of 

 roots produced would be a superastronomical figure: 265X10^^" 

 miles.^ The dry matter contained in these roots would be equal to 

 about 7x10-'^^ times 2 the mass of the earth. This represents an 

 extent of root system of which any tomato plant might well be proud. 



The continuance of growth for such a long period of time proves 

 that the development of the excised tomato root does not depend 

 upon any growth substance or hormones which might have been 

 supplied by the root tip from which the culture was originally 

 started. Since each of the branch roots produced is capable of 

 growth at a rate equal to that of the original explant it must be 

 assumed that these branches have received a portion of any hypoth- 

 ecated essential growth factor initially present. A simple cal- 

 culation shows, however, that even if the original root tip consisted 

 exclusively of such growth hormone molecules, by the end of 5 

 months of weekly subculturing there would be more subcultures than 

 the original number of molecules. 



The excised tomato root was thus shown to be capable of com- 

 pletely satisfying its requirements from the materials furnished in 

 the nutrient solution. It then became of interest to determine just 

 what the minimum requirements might be. It should be realized 

 that the solution of this problem is exceedingly laborious. In addi- 

 tion to the large number of combinations of nutrient media which 

 must be studied when a dozen or more components are involved, there 

 exists the possibility that the value of a given constituent may vary 

 according to the presence or absence of other nutrients. A jBnal 

 answer must be approached by a series of successive approximations 

 and generalized conclusions should be reached with considerable 

 caution. The complexity of the situation has given rise to serious 

 disagreement among various investigators. With better under- 

 standing of the numerous factors which are concerned these differ- 

 ences will doubtless be resolved. 



i265xi0=o«=265 followed by 200 clphera. 

 » 7X10203=7 followed by 263 ciphers, 



