You select any plant you want and if the plant is mature it will have pollen available in the left handle panel. You can use that pollen on that plant, or drag to a different plant to cross pollinate.
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Vista™ Home Premium (6.0, Build 6000); AMD Athlon 64 X2 Dual Core Processor 4800+, ~2.5GHz; 1918MB RAM; DirectX 10; NVIDIA GeForce 6150SE nForce 430 128MB RAM; 1024 x 768 (32 bit) (75Hz)