REPRODUCTION 



347 



and by other cultural methods, one may produce in the plant either 

 an accumulation of carbohydrates or on the contrary an increase 

 in nitrogenous compounds, thus making it possible to control to a 

 certain extent the development and behavior of plants. The 

 results obtained by Klebs have been confirmed lately by Kraus 

 and Kraybill's experiments with fruiting of tomatoes. Moreover, 

 many investigators have observed that an artificial increase of the 

 carbon-dioxide content in greenhouses will facilitate fruiting, since 

 in such case the carbohydrate content of the plant is considerably 

 increased. A number of methods used by fruit growers, for the 

 purpose either of accelerating the flowering or making fruit pro- 

 duction more abundant, are also based on 

 the relationship between carbohydrates and 

 soil nutrients. Transplanting, the cutting 

 of roots, or a temporary neglect of soil 

 tillage are a few of such practices. Such 

 procedures, however, have the disadvantage 

 of curtailing the general development of a 

 plant. Some methods serve to check the 

 removal of assimilates from the stem or the 

 individual branches. This may be accom- 

 plished by girdling of the bark or subjecting 

 it to a pressure by means of a wire, or by 

 partial twisting or breaking of the branches. FlG 144 —Tree girdle on 



Probably the most effective method of a lilac branch. Note callus 

 j. ., r 1 .1 r ,1 above the girdle (after 



increasing fruitfulness is the use of the Moli&ch). 



tree girdle, which is being widely applied 



in Germany (Fig. 144). This girdle consists of a thin strip of zinc, 

 which is firmly fixed around the bark of the stem by means of 

 wire, thus producing pressure and interfering with the downward 

 flow of organic substances. The swelling formed above such a 

 girdle is an evidence of this fact. A belt of this type does no 

 excessive injury to the bark. It can be removed when not 

 wanted any more. The general increase in fruitfulness due to 

 grafting should be attributed, at least partially, to an interference 

 with the downward flow of organic substances at the constricted 

 places. 



The water relations in plants also have a certain effect on the 

 determination of the time of fruiting. A dry air usually favors 

 fruiting, while high humidity delays it or suppresses it entirely. A 



