Page:Popular Science Monthly Volume 77.djvu/349

Rh present in normal quantity although a certain excess will not be harmful. If nitrates are present in excess, however, vegetative growth will be over stimulated and seed production will be small. A lack of phosphates 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 contain just the right amount of nitrates for normal growth and a generous supply of potash and phosphates. The exact amounts must be determined 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 Solanaceæ, the petunias, browallias, 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 comparative 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 protected 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 protection and allows transpiration.

But enough of the technique of hybridizing; the phenomena attending 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 accomplished. From this combination the seed and later the hybrid plant results, half of its characters coming from the plant which