PLANT-TISSUE CULTURES — WEINTRAUB 



367 



this work it would seem that we have another possible answer to 

 the old problem of what makes the sap rise in tall trees. 



One would like to know a great deal more about this phenomenon : 

 the nature of the force responsible for the secretion, for example, 

 and the composition of the secreted liquid; but these must all await 

 future research. Attention should be called to the very interesting 

 rhythmic character of the secretion curve. The rate of secretion con- 

 sistently exhibits a clear diurnal rhythm, being high during the day 

 and falling off nearly or quite to zero at night. Since the roots 

 were cultured in natural daylight it is interesting to speculate that 

 the rhythm is related to illumination, although the tomato roots do 

 not contain chlorophyll and their growth is unaffected by light. 



20 30 



40 

 Hours 



50 



60 



70 



80 



B'lODRB 5. — Curves showing the rates of secretion of two similar excised tomato roots, one 

 at uniform (atmospheric) pressure, the other against imposed pressures up to 90 pounds 

 per square inch. Note the diurnal rhythm exhibited by the curves. Courtesy of Dr. P. R. 

 White (Amer. Journ. Bot., March 1938). 



The present discussion so far has been confined to roots. In a 

 strict sense these root cultures are not tissue cultures at all but rather 

 they are organ cultures. It has been mentioned above that a great 

 deal of the earlier research in the field of plant-tissue cultivation 

 was concerned with attempts to culture isolated cells and fragments 

 of other tissues such as parenchyma, epidermis, and various types of 



