PLANT-TISSUE CULTURES — WEINTRAUB 



361 



It is becoming increasingly evident that the roots of various 

 species of plants may differ sharply in their nutrient requirements 

 and that, in the present fragmentary state of our understanding, 

 generalizations are unjustified. It is of interest that, while some 

 success has been had with roots of about two dozen dicotyledonous 

 species, not a single monocotyledonous root has been grown contin- 

 uously despite numerous attempts. 



Lest a misleading impression be created it should be emphasized 

 that the root cultures so far discussed are in no sense root systems 

 but merely rootlets. As they age, these excised root cultures do not 

 exhibit secondary thickening, as do normal roots, but remain indefi- 

 nitely in a juvenile condition. Typical cultures of this kind are 

 illustrated in figures 1 and 2. 



In unpublished work of the author a somewhat different objective 

 has been considered. It has been attempted to produce an excised 

 root system resembling that normally developed by the intact plant 



FiGDRH 1. — Excised root of tomato. The fragment with which the culture was started can 

 be distinguished near the center of the photograph. Courtesy of Dr. W. J. Robbins (Bot. 

 Gaz., June 1938). 



