8 N. H. Agricultural Experiment Station [Bulletin 274 



dark days about the time fruit-bud formation usually takes place. 4. Im- 

 proper balance between food and nutrients, which may be caused by lack of 

 leaf surface due to insect attacks or lack of minerals in the soil, especially 

 nitrogen. Once flower buds have been initiated, conditions for carbohy- 

 drate, moisture, and mineral supplies must be kept as near the optimum as 

 possible to insure continued development of the buds. 



The tree must also be brought safely into the rest period by prevention 

 of growth late in the season. Late growth may result from injudicious 

 applications of nitrogen or excess moisture in the soil. In the cultivated 

 orchard a cover crop planted about July 1 will alleviate the latter condition. 



In the winter there is danger with the more tender varieties that 25° to 

 30° F., below zero will freeze some or all of the flower buds. Once killed by 

 freezing, no other flower buds will form in time to open in the spring. The 

 leaf buds are more hardy and do not freeze at these temperatures. Provided 

 the hazards of winter have been avoided, there is still danger from spring 

 frost where a temperature of 25° or 26° F. may totally ruin the fruit crop 

 through freezing of the unfolding buds or open blossoms. Under such con- 

 ditions the pistils are usually injured. The pollen may also be injured and 

 often the ovules. 



To insure the proper nutrient and moisture conditions, an appHcation of 

 nitrogen is generally made in the sod or sod-mulch orchard a short time 

 before the trees come into bloom in May and a mulch applied a little before 

 July 1 to conserve moisture. This early nitrogen application is beneficial 

 to the setting of fruit, to fruit-bud formation, and to the further develop- 

 ment of the fruit. Light frost after the buds have appeared may not inter- 

 fere at times with the set or pollen formation but may cause injury to the 

 outer layer or epidermis of the flower parts. The result will be a russeted 

 fruit or one with russet rings, cracks, or malformations. 



III. Mechanics of Apple Fertilization 



Pollination: 



Pollination means the transfer of pollen from the stamens to the pistils 

 of the flowers. With apples this is mostly eft'ected by insects, principally the 

 various kinds of bees. The pollination of apples cannot be assured through 

 the agency of the wind. 



Pollination is only one step toward insuring a set of fruit. FertiHzation 

 of the egg cells in the ovules is necessary with most apples to cause fruit to 

 set. This fertilization process results in the formation of seed. Further 

 growth of the tissue of the fruit is directly correlated with seed develop- 

 ment. The pollen of only certain varieties can under certain conditions 

 effect satisfactory fertilization and set, hence the need to study the value of 

 difl"erent varieties as pollenizers for the different apple sorts. 



To show the relation of various factors to the fertilization of flowers and 

 set of fruit, a short description of the structure of the flower and the pro- 

 cess of egg fertilization is presented. 



The Flower: 



Fig. 4 shows the cross section of an apple flower. Figs. 5, 6 and 7 show 

 and describe the essential parts in detail. The important parts of the flower 

 with relation to pollination are the stamens and pistils. Each stamen con- 



