6. The following experiment was undertaken to determine if the pollen tubes obtain nutriment from the silks as they grow downward or whether food materials stored in the pollen grains are the chief source of energy.
Pollinations were made one day after cutting back the silks, so that brushes of silks approximately 1-1/2 inches long were available. Following pollination that portion of the silk (with the attached pollen grains) extending beyond the husks was cut off at intervals of 1/2, 3/4, 1, 1-1/2, 2, 2-1/2, 3, 3-1/2, 4, and 6 hours after pollinating. Silks removed at different intervals of time were fixed in alcohol and later stained with carmine chloral hydrate.
It was found that germination occurred within the first half-hour. Germinated grains on silks removed at the different time intervals were examined cytologically to determine whether or not the two sperm cells and the tube nucleus had passed into the silk. The data are given as follows:
Table 1. Percent of germinated grains with no (0), one (1), and two (2) sperm nuclei, and having (1) or lacking (0) a tube nucleus on silks removed at different time intervals after pollination.
|2 sperm cells
1 tube nucleus
The average number of grains on each examined silk was approximately twenty but considerable variation was found. Every silk examined, however, had a number of established grains.
Most of the sperm and tube nuclei pass out of the pollen grains between one and two hours after pollination. The sperm cells usually precede the tube nucleus in passing into the pollen tube. Four hours after pollination the pollen grains are nearly empty. The pollen grains retained a considerable portion of their contents two hours after pollination, even though the sperm nuclei and the tube nucleus had entered the silk. Pollen grains cut off before all of the food reserves had passed into the pollen tubes might not achieve fertilization for lack of sufficient nutriment if the growing tubes obtain little or no nourishment from the stylar tissue. The pollen tubes would contain the sperm and tube nuclei, but only part of the total food material stored in the pollen. If the pollen tubes obtained nutriment from the silk, they would continue to grow and all the ovaries would be fertilized. If, however, the pollen tube could not obtain sufficient nutriment from the silk, it would grow only until the available food material in the pollen tube was exhausted. Many of the ovaries at the bottom of the ear would not be fertilized, because the pollen tubes lacked the energy to grow a longer distance.
Seed set was determined at maturity.
Table 2. Number of ears, total number of seeds, and the percent of seeds found in the upper half of all the ears of corn for each time interval.
|Series A||Hours after Pollination|
|Number of ears||3||6||5||9||10||4|
|Total no. of seeds||1||193||862||337||1233||2607||1380|
|Percent seeds in upper half||-||64||71||75||72||58||52|
|Series B||Hours after pollination|
|Number of ears||8||8||8||7||11|
|Total no. of seeds||5||26||81||408||3182|
|Percent seeds in upper half||-||78||69||68||52|
|(Note: ∞ = silks were not removed)|
The number of seeds in the upperhalf of the ear was consistently greater than in the lower half at the time intervals when food material still remained in the pollen grain at the time of removal. Inasmuch as nearly all of the contents of the pollen grain had been discharged into the pollen tube by four hours after pollination but there were an appreciable number of unfertilized ovules at the base of the ear it seems that practically all of the stored reserves are needed for the long journey to the basal ovules. it is doubtful if the stylar tissue offers any nourishment to the growing pollen tube.