PRINCIPLES OF HEREDITY 13 



was made in the improvement of plants until within 

 the last fifty years. 



16. Artificial Crossing. It is a very easy matter to cross 

 plants. The essential steps are: 



(1) Prevent undesired pollen from reaching the stigma. 



(2) Apply the desired pollen when the stigma is ready 

 to receive it. 



Suppose it is desired to cross a Ben Davis and a Winesap 

 apple. Shortly before the blossoms open, remove the 

 stamens from several blossoms on one tree, say the Ben 

 Davis, and cover with paper bags. Care must be taken 

 not to injure the pistil. The petals may be removed if 

 they are in the way. (Fig. 6 shows a pea blossom with 

 petals and stamens removed.) When the blossoms on 

 the tree are in full bloom, it is ready for the application 

 of pollen. Jar a number of blossoms on the Winesap tree 

 over a small dish, so as to get pollen from them. This 

 may be applied to the stigmas of the blossoms on the Ben 

 Davis tree with a camel's-hair brush or with the finger. 

 Cover with the paper bags to prevent other pollen entering. 

 Remove the bags after the fruit is set. In the case of corn, 

 the ears are covered with bags when the silks appear, 

 and, later, pollen is applied from the desired tassel. 



When two varieties or breeds of plants or animals 

 are crossed, the new individual is called a hybrid. 1 



SOME PRINCIPLES OF HEREDITY 



17. Problems of Heredity. If a pollen grain from a red- 

 flowered pea fertilizes an ovule from a white pea, some 



'The word hybrid was formerly restricted to crosses between different 

 species, but it is now commonly used to designate any cross. 



