PLANT BREEDING 343 



present in normal quantity although a certain excess will not be harm- 

 ful. If nitrates are present in excess, however, vegetative growth will 

 be over stimulated and seed production will be small. A lack of phos- 

 phates will produce the same effect upon seed production, but for a 

 different reason. Phosphorus is an essential constituent of the proteid 

 compounds found in large quantities in the seed. Therefore, if the 

 plants are to be in the best condition for crossing, the soil should con- 

 tain just the right amount of nitrates for normal growth and a generous 

 supply of potash and phosphates. The exact amounts must be deter- 

 mined by experience for each soil and each species of plant. 



Other necessary knowledge that can be obtained only from experience 

 is which are the best flowers on the plant to serve as parents of the cross 

 and what is the proper time for their pollination. For example, in the 

 grasses the first flowers that appear usually form larger, healthier seed 

 than the later blossoms. In most of the Solanaceas, the petunias, browal- 

 lias, etc., the exact opposite is true. The time when the individual 

 flower is most receptive to pollen is even more narrowly limited. Both 

 premature and delayed pollination is the cause of many failures and the 

 optimum time should be accurately determined. Having exercised these 

 precautions, it remains to study carefully the structure of the flower 

 in order that it may be emasculated — i. e., the anthers removed before 

 the pollen is shed — with sufficient adroitness that neither the anthers 

 shall be opened nor the parts of the pistil injured. Only a few buds 

 upon a single flower spike should be operated upon if they are to be 

 given the best chance of development. If the buds are very small and 

 some pollen unavoidably reaches them, it may be washed off with com- 

 parative safety with a dental syringe if done immediately. It is often 

 recommended that the calyx and corolla be cut away when emasculating. 

 This should be avoided if possible and the floral envelopes left as a 

 protection to the pistil. After emasculation the buds should be pro- 

 tected from foreign pollen until time for pollination, and again after 

 pollination at least until the fruits have begun to form. This protection 

 may be an ordinary paper bag when the crossing is done in the field. 

 In the greenhouse I find that a square of thin celluloid rolled around 

 the flower and caught with two rubber bands, each end being protected 

 with absorbent cotton plugs, is a better device. It gives excellent pro- 

 tection and allows transpiration. 



But enough of the technique of hybridizing ; the phenomena attend- 

 ing it are of more importance. After the pollen is placed upon the 

 stigma it begins to grow until it reaches the ovule. Down this tube 

 comes the male cell which contains the potentialities of its parent plant. 

 This cell fuses with the female cell in the ovule and fertilization is ac- 

 complished. From this combination the seed and later the hybrid 

 plant results, half of its characters coming from the plant which fur- 



