HYBRIDS 129 



accept pollen which is furnished under unusual con- 

 ditions. 



Plant breeding was introduced at the beginning of 

 the nineteenth century by Andrew Knight, William 

 Herbert and Carl Friedrich von Gaertner, but their 

 work was not appreciated, and in fact was over- 

 looked, until Mendel's papers were given attention 

 nearly a century later. Scientific plant breeding be- 

 gan with full force in the beginning of the twentieth 

 century and is now proceeding with great rapidity. 



The method of crossing flowers is simple enough 

 but one must guard against accidents. Male flowers 

 are to be plucked from a branch carrying female 

 flowers and the branch is then covered with a paper 

 bag before the male flowers have begun to shed their 

 pollen on that particular tree or before pollen has 

 been blown from neighboring trees of allied species 

 and varieties. The reason for this is that pollen is 

 carried through the air for a considerable distance 

 by the wind. If any flowers of trees of allied species 

 or varieties in the vicinity are shedding pollen some 

 of this pollen may already have alighted upon fe- 

 male flowers which we wish to hybridize. For pro- 

 tection purposes ordinary paper bags of the sort to 

 be purchased at the grocery store will suffice, but 

 some hybridizers prefer to oil these bags because 

 the oiled paper allows more light to enter and lessens 

 the possibility of any minute pollen grains making 

 their way through imperceptible cracks in the bag. 

 For most nut trees paper bags of ten, fifteen, and 



