Stimulation of Root Growth in Cuttings 



103 



may partially explain the fact that parts of tops, such as cuttings without 

 roots, are more quickly injured in a soil with a high organic and carbon- 

 dioxide content or a low oxygen content than are rooted cuttings or seed- 

 lings. It is certainly true that cuttings are less tolerant of poor aeration. 

 Kidd (1914) found that a rise in temperature lessened the inhibiting 

 effect of carbon dioxide. This might partially explain the beneficial 

 effect of bottom heat, which is discussed later. 



In a few preliminary experiments with cuttings inclosed in chambers 

 with increased oxygen content and in chambers with increased carbon- 

 dioxide content, respectively, the cuttings in the increased carbon-dioxide 

 content were distinctly injured, while those in the increased oxygen con- 

 tent remained healthy much longer than did cuttings in normal inclosed 

 air. In none of these preliminary experiments did any of the Ligustrum 

 cuttings remain healthy long enough to form roots. Some very suggestive 

 results, however, were obtained by placing twigs under a suction pump, 

 reducing the pressure by suction, and then replacing the air with carbon 

 dioxide or oxygen. Results from such treatment are illustrated in table 8 : 



TABLE 8. Effect on Root Growth when the Gas in Twigs op Lioustrum is 

 Replaced with Oxygen or Carbon Dioxide 



(Cuttings taken on March 29, measured on June 2, 1917. Ten twigs to the culture) 



*0nly one twig in each culture waa rooted; the others had very weak calluses or none at all. 



The twigs were placed under suction for twenty minutes and then allowed 

 to stand in the gas at atmospheric pressure for sixteen hours. They 

 were then removed and placed in flasks of tap water. The single injection 

 of carbon-dioxide gas distinctly retarded both root and top development. 



