GROWTH 



291 



favorable for it. One cannot make the potato tuber sprout in 

 September and October even under favorable temperature and 

 moisture conditions, while in spring it will develop in a cold dry 

 room. 



Growth of dormant organs, therefore, is not controlled by 

 external inhibiting factors. It is not known what these factors are, 

 but there are reasons to suppose that the presence of some sub- 

 stances opposing growth exist, the nature of which is not yet under- 

 stood. Still, there are several means of removing these inhibitors 

 and of inducing resting organs to resume growth. Some of these 

 methods have found wide application in horticulture for the forcing 



Fig. 113. — Lily-of-the-valley treated with ether, A, and control, B (after Duggar). 



of early blossoming of perennial plants. The most important of 



these l is a treatment with ether, discovered by Johannson, and 



the exposure to warm baths suggested by Molisch. 



Plants intended for treatment with ether, such as shrubs of 



lilac, rhizomes of lily-of-the-valley, etc., are placed in air-tight 



wooden boxes with closely fitting doors into which at the same time 



is placed a small cup of ordinary ether. The concentration of ether 



is usually 0.5 cc. per liter of air. The plants are allowed to stand 



in this vapor for 24 or 48 hr. after which they are transferred to a 



warm greenhouse. Soon the treated plants will open their buds 



1 In America, the more effective treatments by ethylene, ethylene chlor- 

 hydrin, and similar compounds are used. 



