52 POMOLOGY 



48. The periodic idea. — Closely related to the internal 

 factors referred to above is the time at which trees reach the 

 bearing age. In the first place, as has been noted, a given 

 variety of fruit-tree has a tendency to bear at a definite age: 

 the Yellow Transparent apple bears at about four years of 

 age; the Mcintosh at five or six; the Baldwin at nine or ten; 

 while the Northern Spy may be twelve or fifteen years or 

 older before fruiting is established. To such a characteristic 

 botanist? have given the term "periodicity," although it 

 usually refers to the periodic phenomena of a single plant. 

 Jost says, "When we examine flowering plants under natural 

 conditions we find that the plant is 'ripe for flowering' just 

 as sexual cells appear in the animal when it reaches a certain 

 age. But although flower formation, generally speaking, 

 takes place at a certain age, which diiTers with each species, 

 still, exceptions are known, as for example, the oak, which 

 normally is ' ripe ' in its sixtieth year, but which occasionally 

 flowers in its first year and then dies, " i. e., environment may 

 alter this particular periodic effect.^ While this does not 

 explain the causes for the flower-bud formation, it is a 

 manifestation that should be recognized. 



49. Theory of specific constructive materials. — This 

 theory, which has been proposed by at least one plant 

 physiologist (Sachs) and rejected by some of the others, 

 may be given brief mention, although it relates to plants in 

 general and has no special reference to fruit-trees. 



In attempting to explain why flower-buds are differentiated 

 from leaf-buds, Sachs suggested that, in addition to the 

 products of assimilation in the leaf, there are also specific 

 constructive materials which pass from the leaf in all direc- 

 tions and which collect in certain quantities where a definite 

 organ is to be developed. Thus, the flower would be formed 

 out of flower-building material, roots out of root-building 

 1 Jost, L. Ibid., p. 363. 



