396 BOTANY OF CROP PLANTS 



On its edge, are arranged five sepals, five petals, and fifteen 

 to twenty stamens. There is a single pistil bearing one style 

 and one stigma. The pistil is at the bottom of the recep- 

 tacle. There are two ovules in the young ovary; one of them 

 aborts during maturation of the fruit. 



Fertilization. — Many of the plums are practically self- 

 sterile. The native plums exhibit the greatest self-steriUty; 

 this is due to the impotency of the pollen when used on the 

 stigma of the same flower. Japanese and domestic plums are 

 less self-sterile than native species. In some cases, not only 

 are pistils developed that are so weak as to fail even if polli- 

 nated, but some flowers do not form pistils. Again, pistils and 

 stamens of the same flower often mature at different times. 

 Usually, the pistils mature first. Rarely, the opposite is 

 the case. Hence it is seen that cross-fertilization is very nec- 

 essary in plum orchards, but not only cross-fertilization be- 

 tween different trees of the same variety but between dif- 

 ferent varieties. It is reported by Hendrickson that French 

 and sugar prunes in CaUfornia set a very light crop unless a 

 large number of bees are present in the orchard at the time 

 of blooming. They appear to be self-sterile to some extent. 

 Imperial prune trees that were enclosed in a tent from which 

 all insects were excluded set no fruit. It seems that, with the 

 Imperial prune, fruit is not set unless pollen is brought from 

 other trees. It is distinctly self-sterile. All Prunus species 

 are insect-polKnated for the most part. 



Fruit. — After fertilization, the receptacle, with its attached 

 sepals, petals, and stamens, is cut off by a circular abscission 

 layer near its base (Fig. 164). The ovary wall increases in 

 thickness to form the following fruit parts (Fig. 164): (i) 

 skin, exocarp; (2) flesh, mesocarp; and (3) hard stony layer 

 about the seed, endocarp. The style and stigma do not per- 

 sist in the fruit. The seed is within the endocarp. Hence 



