Sentence:  Orchids rely on fungi to reproduce. Their tiny seeds don't have any on-board nutrients (like beans and apples) and will not <WSD>germinate</WSD> until they are infected by a symbiotic fungus which supplies them with food. Known as a protocorm, this tiny orchid-fungus ball grows, turns green and eventually starts to photosynthesise. 
Meaning of the word marked with <WSD> tags:  Of a seed, to begin to grow, to sprout roots and leaves.