284 POMOLOGY 



following may be listed as the more common causes: failure 

 of fruit-buds to develop; lack of vigor or excessive vigor re- 

 sulting in the dropping of the expanded flowers; winter in- 

 jury to the floral parts; frost at blossoming time resulting 

 in injury to flowers or inactivity of pollen-cany ing insects 

 and the consequent lack of pollination; lack of "affinity" 

 between varieties (self- or inter-sterility); defective pollen 

 or embryo sacs; or hybridity, the causes of which are not 

 understood. 



Some of these factors are treated elsewhere and only the 

 various problems having to do with pollination and sterility 

 will be considered at this time. 



254. Development of pollen.^ — The pollen produced 

 within the anther-sacs of the stamens contains the male ele- 

 ment of the reproductive system of flowering plants. A 

 brief statement of the development of the pollen will more 

 clearly introduce the problems encountered in a study of 

 pollination. A sufficient similarity exists between the several 

 tree-fruits that all need not be considered. 



The stamens, as indicated in Chapter III, originate as 

 an outgrowth from the torus, differentiating in their devel- 

 opment into filaments and anthers. The anthers consist 

 essentially of four lobes of pollen-forming tissue which, on 

 further development, become differentiated as four sacs or 

 locules, each of which contains the pollen-grains, one sac 

 corresponding to each lobe. In the early development of 

 the anther, the sporogenous tissue is first seen within each 

 lobe, and surrounding the future ''pollen-making" tissue is 

 a layer of cells known as the tapetum. The cells within 

 the tapetum become the mother cells which, on further di- 



1 See Sandsten, E. P. Wis. Agr. Exp. Sta. Res. Bull. 4. 1909. Kraus, 

 E. J. Ore. Agr. Exp. Sta. Res. Bull. 1, Part 1. 1913. Dorsey, M. J. 

 Minn. Agr. Exp. Sta. Bull. 144. 1914. Black, C. A. N. H. Agr. Exp. 

 Sta. Tech. Bull. 10. 1916. 



