Why is heartwood darker in color than sapwood?

The influenza virus cause the flu, but not other diseases. why?

How can antibodies "remember" a particular bacterial invader or "antigen" on that invader?

Select the correct answer from each drop-down menu. during the day, plants produce by splitting water molecules in the light-dependent reactions of photosynthesis. at the same time, plants use cellular respiration to produce some of the needed by the light-independent reactions to make sugars. during the night, plants produce because takes place.