THE INITIATION OF THE REPRODUCTIVE PROCESSES 193 



only during cold weather and is resumed whenever temperatures permit. 

 Many of these blossoms are killed by cold. 



Magness^" noted a difference in the stage of development of buds on 

 spurs in successive years. Buds of the Tetofski apple in November, 

 1914, showed ovules developed, while in December, 1915, they had not 

 reached that stage. 



"The blooming season during the spring of 1915 was fully one week earlier," 

 he states, "than that of 1916. It is quite probable that factors operating during 

 the late summer and fall to hasten or retard flower development, as well as factors 

 operating during the spring, materially influence the time of blossoming in our 

 orchard fruits." This statement is of particular interest when correlated with 

 Sandsten's work, discussed under Temperature Relations. 



Summary. — The available data do not permit a definite statement of 

 the exact cause or causes of fruit bud differentiation or an exact de- 

 cription of the internal nutritive conditions associated with fruitfulness 

 and unfruitfulness. However, there must be at least two antecedents 

 to an initiation of the reproductive processes: (1) There must be an 

 excess of carbohydrates above the amount required for vegetative 

 development. The rate of manufacture must exceed the rate of utili- 

 zation. (2) There must not be any limiting factor that entirely stops 

 vegetative growth which must continue within the bud even though there 

 be no new shoots and leaves formed or even no visible indication of an 

 increase in the size of the buds that are differentiating flower parts. In 

 Jthe orchard the supply of available nitrogen is probably the most common 

 limiting factor. If nitrogen is present in large amounts it forces the rapid 

 utilization of carbohydrates so that their accumulation cannot occur. If 

 it is very limited in amount, growth is practically stopped before fruit 

 bud differentiation can take place. Carbohydrate accumulation may 

 not in itself be the cause of the fruitful condition in the plant as a whole 

 or in its individual parts. It may simply be another result of the same 

 factors that lead to fruitfulness; at least, however, the two are associated. 



In practically all the deciduous fruits growing in temperate climates 

 fruit bud differentiation occurs during the summer or fall previous to the 

 opening of the buds. Every bud that is formed may be considered a 

 potential fruit bud, but practically differentiation takes place only when 

 suitable nutritive conditions are provided. Ordinarily each bud develops 

 to a certain point and then comes to a comparative rest. Later develop- 

 ment is as a vegetative bud or a flower bud, depending on whether con- 

 ditions do or do not favor differentiation of flower parts in the slow growth 

 that takes place during the period of comparative rest. The exact time 

 of differentiation varies considerably with variety, seasonal conditions, 

 moisture supply, method of culture, position on the plant and other 

 factors. In cold climates there are practically no changes within the bud 

 during the winter. 



