PHYSIOLOGY OF THE DEVELOPMENT OF PLANTS 415 



or a temporary neglect of soil tillage are a few of such practices. 

 These procedures, however, have the disadvantage of curtailing 

 the general development of a plant. More reasonable are the 

 methods that serve to check the removal of assimilates from the 

 stem or the individual branches. This may be accomplished 

 by girdling the bark or subjecting it to pressure by means of a 

 wire or by partial twisting or breaking of the branches. Proba- 

 bly the most effective method of increasing fruitfulness is the use 

 of the tree girdle, which is popular in Germany (Fig. 130). 

 This girdle consists of a thin strip of zinc, which is firmly tied 

 around the bark of the stem by means of 

 wire, thus producing pressure and interfer- 

 ing 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 permanent injury 

 to the bark. It can be removed when not 

 wanted. The general increase in fruitful- 

 ness due to grafting should be attributed, 

 at least partly, to an interference with the 

 downward flow of organic substances at the 

 graft union. 



The water relations in plants also have a on'l' maf bi^iTch.^ N^ote 

 certain effect on the determination of the callus above the girdle 



i- rr'i.' Ai • nr {after Molisch). 



time oi iruitmg. A dry air usually lavors 



fruiting, while high humidity delays or suppresses it entirely. 

 A dry soil, on the other hand, appears to be less favorable to 

 fruiting than a humid one. Consequently, the conditions that 

 are most favorable to fruiting are those attained on well-irrigated 

 land in an arid climate with a large number of bright days, as 

 for instance in California or Central Asia. Under such con- 

 ditions, fruit trees generally yield very large crops. 



Klebs's theory of the significance of the relationship between 

 carbohydrates and nitrogenous substances has become popular 

 in America. By studying the question of fruiting of tomatoes, 

 Kraus and Kraybill (1918) have contributed much to the estab- 

 lishment of the significance of this concept. During the last 

 10 to 15 years, it has been shown that other proportional ratios of 

 nutrients are just as important as the C/N ratio alone. Although 

 it exerts a certain influence on the abundance of fruiting, which 



