added protocol

assays/complementation_crosses/protocols/crossing_plants.md

+**crossing of plants**
+
+- grow parent plants in LD conditions
+- if parental plants have very different flowering times: sow the faster one with a bit of delay)
+
+
+- emasculate florets from mother plants before pollen are shed (about Waddington stage 8.5 - 9.0)
+- pollinate florets with pollen from father plants, pollinate all florets from one spike with the same father: this is one cross
+- cover the spike with a paper bag, count later how many of the pollinated florets develop into seeds
+- harvest seeds from each spike / each cross individually when they are completely dry
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