black: inflorescence and fruit of pyrus malus 523 



according to structure as leaf, flower, and mixed buds. The flower 

 bud of the apple contains both flowers and leaves and is therefore, 

 properly, a mixed bud, as shown in the expanded buds in plate 33, 

 FIGURES 7 and 8, and plate 34, figure i. 



The time of flower bud formation is somewhat variable ac- 

 cording to the general conditions of growth and the state of the 

 apple tree and is either in the spring when growth is resumed or 

 the buds may develop on the second growth of the season if such 

 growth occurs. An interesting article on the conditions affecting 

 the flowering of fruit trees by Sandsten (5) states that a physio- 

 logical constant can be formulated from the climatic conditions 

 during the ten months preceding flowering. He concludes that:^ 

 "The time of flowering in the spring of a certain variety of fruit 

 is dependent upon a number of causes or conditions chief among 

 them being, first, the number of positive temperature units 

 received in the spring preparatory to flowering; second the stage 

 of development of the flower buds as dependent upon the climatic 

 conditions of the summer and fall preceding the flowering; third 

 the fruiting of the trees, — whether light or heavy the year previous 

 to flowering; fourth, soil conditions and the amount of plant food 

 present in the soil; and fifth, the individual characteristics and 

 state of health of the tree and plant." By the time the flowers 

 have advanced to the stage shown in plate 33, figure 8, and 

 PLATE 34, figure I, onc or more well-established growing points 

 may be found in the axils of the lower leaves on the "purse." 

 These growing points are the primordia of the buds for the follow- 

 ing year. An examination of trees in the middle of July revealed 

 apples approximately two inches in diameter on a well-developed 

 "purse." The bud for the next year was well established. It 

 was partially surrounded by the petioles of two or more leaves. 



The time of the differentiation of flower buds has been suggested 

 by Kraus (6) for Oregon by the statement that the microsporangia 

 pass the winter in the mother-cell stage. Drinkard ( 7) for Virginia 

 states that the primordia of the flowers were found in July and by 

 December the flower parts were completely formed. In February, 

 resting pollen-mother-cells were found. In Wisconsin Goff (8) 

 gives the date of June 30 for the earliest evidence of flower pro- 

 duction. He states that in the apple a longer time apparently 



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