456 THE BIOLOGY OF FLOWERING PLANTS 



the carbohydrates increased towards the beginning of the 

 rest period, and that compounds of phosphorus and 

 nitrogen increased towards the end of winter. Howard 

 (19 1 5) found enzymes much more active at the end of the 

 rest period than during it. 



Forcing. — Many other methods of overcoming the rest 

 period are known. Miiller-Thurgau (1885, 19 12) found that 

 potato tubers could be forced to immediate development 

 by subjecting them to cold after harvesting — an imitation 

 of the natural process in winter. Johannsen (1906) forced 

 the lilac and other shrubs by exposing them to ether vapour 

 for one or two days. Molisch (1909, 1921) forced lilac, 

 hazel, willow, and others by plunging them for twelve 

 hours in a bath of warm (30° to 35° C.) water, and the horse 

 chestnut by exposure to radium emanations. Jesenko 

 (191 1, 1912) forced various trees by baths or injections of 

 alcohol, acids, and water, and by pressing water into the 

 cut surface of the shoot. Weber (19 16) forced the beech 

 by acetylene. Generally it is impossible by these methods 

 to force with success during a middle period of rest, which 

 occurs in September, October, or December ; but, as we 

 have seen, Klebs forced the beech at any time (though with 

 difficulty in November), and Lakon (191 2) forced various 

 trees at any time by placing the cut shoots in a nutrient 

 salt solution. The ether and the warm bath methods are 

 of economic importance. 



These various methods are, of course, unnatural and 

 often drastic. In no case has their action been analysed, but 

 they may very well agree with Klebs's theory that an activa- 

 tion of enzymes is important. While the question of the 

 cause of the resting period and of rhythm in vegetative 

 development is not by any means settled, it may be said 

 that a possible explanation lies in the interaction of external 

 factors with the conditions of growth and development at 

 the vegetative point, even when the external factors are 

 apparently very uniform. We must note that the production 

 of leaves is itself a periodic phenomenon, as is, to go a step 

 further back, cell division also, the basal fact in develop- 



