456 



PLANT PHYSIOLOGY 



104. Artificial Breaking of the Rest Period. Forcing of Plants. 

 Significance of This Method for Agricultural and Floricultural 

 Practice. — The causes that inhibit the growth of dormant plant 

 organs are not known. Many authors suppose the presence of 

 hypothetical growth-inhibiting substances. For instance, the 

 dormancy of lateral and accessory buds and their rejuvenation 

 after the removal of the terminal bud are explained by the 

 assumption that the growing point sends downward hormones 

 that inhibit cell division. After removal of the top bud, the 

 inhibiting influence ceases. 



Fig. 141. — Lily of the valley treated with ether, A, and control, B {after Duggar). 



In this case, there is a manifestation of correlation between the 

 separate parts of the plant, which w^as discussed in Art. 91. 



But even if the internal mechanism of dormancy is not yet 

 understood, nevertheless there are many means of breaking it 

 artificially. Most of these methods have originated in horti- 

 cultural practice, and their perfecting has been promoted by 

 the desire to obtain flowering plants in winter, which are highly 

 valued but difficult to force at this time. Recently, new and 

 improved methods of breaking the rest period have been estab- 

 lished by physiologists. The most important of these is treat- 

 ment with ether, discovered by Johannsen; the exposure to 

 warm baths, suggested by Molisch; and the newer methods 

 practiced in the United States, using more efficient and less 

 toxic chemicals. 



