The first section of a scientific paper (after the abstract) is the Introduction. This is where the researchers provide relevant background information on their topic of investigation. This section shows that the researchers have read and are familiar with the research that has already occurred on this topic. It is full of footnotes or in-text citations (this article uses in-text citations) that inform the reader about the origin of the information (i. e. where that information came from). This allows the reader to look up the original research, if they so desire.

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explanation:

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the dark variety, as they are not visible on the dark tree barks

explanation:

these muscles maintain the blood pressure in the vessels. the smooth muscles are innervated by nerves of the autonomic nervous system. when the muscles contract, they cause vasoconstriction of the vessels and increase blood pressure. when the muscles relax, they enable vasodilation of the   blood vessels hence reduce blood pressure.  

the alkaline earth metals (beryllium (be), magnesium (mg), calcium (ca), strontium (sr), barium (ba), and radium (ra)) are a group of chemical elements in the s-block of the periodic table with very similar properties: shiny. silvery-white. hopefully this ? i didn't fully understand the question