Abstract
The historian studies revealed during Mendel's later research period when mainly focusing on the constant hybrid in Hieracium, he had to be intervened to conduct the controlled pollination experiments in Mirabilis jalapa. Two letters to Nageli recorded the experimental aim was to disprove Darwin's opinion regarding three pollen grains required for one fertilization (note: that could completely destroy his previous discovery of segregation inheritance in variable hybrid in Pisum, for it was expressed in a mathematical equation). The experimental results of single pollen grain pollination confirmed the referenced view of one pollen cell uniting one egg cell in plant fertilization; the further pedigree introduction of the single and of the designed two pollen grain experiment succeeded in exemplifying that one hereditary factor carried by one gamete (pollen cell or egg cell) can independently transmit a trait to offspring. Here we coined the observation as the Gamete Theory of Inheritance. Remarkably, in contrast with the bulked pollination experiment, in this system, Mendel could easily manipulate a hereditary factor by merely taking a gamete as a carrier. Then, Mendel's work in M. jalapa together with the previous Pisum study was able to jointly suppport his second lecture content that regarded "gamete formation, fertilization, and seed development" and also regarded hereditary factors in the processes. All in all, the 1866 paper was published during a rapid burst of interest in hybrid species likely induced by Darwin, and Mendel's attempts at accommodation of the two incompatible inheritances of segregation in variable hybrids versus of nonsegregation in constant hybrids might be responsible for some historical controversies when understanding his discovery of inheritance.