PLANT REPRODUCTION lOl 



tlon ; they are known as parthenocarpic fruits. Some fruit trees 

 will set very little or no fruit unless they have been pollinated 

 with pollen from other varieties. This becomes an important 

 factor in tbe setting of orchard trees and determines whether 

 the grower can set an orchard of a single variety or must use a 

 mixture of varieties in order to secure a crop of fruit. A second 

 function is the effect on the endosperm, which is very evident in 

 corn and is known as xenia. In this case the effect of the second 

 male sperm on the endosperm can be seen through the seed coats. 

 It is well illustrated where two varieties of corn aj^ grown to- 

 gether; some of tbo ovules ha^ang been pollinated with pollen 

 from plants of the same variety and others with pollen from 

 plants of a different variety, results in ears with two types of 

 grains. In most species of plants this xenia character is lost 

 in the seeds and fruits, due to the absorption of the endosperm 

 by the embryo, or to concealment by the coverings. 



Parthenogenesis and Polyembryony. — Some plants, saich as 

 the dandelion, will produce seeds vdthout fertilization; this 

 process is known as parthenogenesis. Seeds occasionally pro- 

 duce more than one embryo ; this may be duo to the formation 

 and fertilization of more than one ovum or to the formation of 

 embryos from nucellar tissue within the embryo sac or both. 

 The seeds of the orange frequently produce several embryosi, 

 one of which may be from a fertilized egg, while the others are 

 from the nucellus. This becomes an important factor in the 

 cross-breeding of related plants for the development of new 

 varieties, since the embryo from the fertilized egg is the only one 

 that will possess character of both parents, the others possessing 

 characters of the mother plant only. 



Non-Sexual Reproduction. — Most of the higher plants can 



