366 THE AMERICAN NATURALIST [Vol. LI V 



ical root system. I have preserved the stems of plants steamed 

 in this way and Fig. 1 is a drawing of one of these. It will 

 be noted that the steamed region, which was exposed for. 

 three minutes in this plant, has shrivelled to a hard woody con- 

 nection about 24 mm. long. Table I gives the data regarding 

 the growth of control and steamed plants whose tops were not 

 killed by the steaming. The average growth for the three con- 

 trols is 40.5 mm., and for the steamed plants, 59 mm. It is evi- 

 dent from the table also that the terminal bud has grown in the 

 24 hrs. immediately after steaming so that we cannot say that 

 the steaming caused even a temporary cessation of growth of 

 the tip. 



TABLE I 



It is certainly true that sap must pass up the stem of these 

 steamed plants, otherwise the tops would remain turgid and 

 growth occur. Root inhibiting substances, if formed, must have 

 passed upward along with the sap. Nevertheless we find roots 

 developing above the steamed region despite the fact that the 

 plant has a normal living root system below. The evidence is 

 conclusively against the existence of definite root inhibitive sub- 

 stances. If sap can pass upward in a steamed area we might 

 expect that it could pass downward also. If inhibitive sub- 

 stances are formed by a growing stem these materials should 

 reach the cotyledonary buds below. Nevertheless these buds 

 develop. Since we can not necessarily argue that because ma- 

 terial can pass up a stem it must also pass down, the evidence 

 points against the existence of shoot inhibitive substances, but 

 is not unequivocal. 



