I think it would be to exchange gases. When you think about it, plants in the past were always underwater. They didn't have any air. Now, they are on land and need to be able to exchange gases from the leaves to the roots.
Animal Kingdom. One of the six kingdoms, characteristics include: heterotrophic (obtains energy from eating other organisms), multicellular, eukaryotic and cells have a nucleus but no cell wall or chloroplasts.
Population decreases by 15.
Birth rate means more population (positive), death rate means less population (negative), immigration means more (positive), and emigration means less (negative). So, just add and subtract the appropriate numbers:

Negative 15 people means that the population decreases by 15.
Answer: Coronary Sulcus
Explanation:
The atria and ventricles are separated externally by deep coronry sulcus.
Coronary sulcus is also known as atrioventricular sulcus which expands around the circumference of the heart. Coronary sulcus are of two types including posterior interventricular sulcus and anterior interventricular sulcus which are present between the right and left ventricles on the posterior surfaces and anterior surfaces of the heart.
These sulcus are made up of adipose connective tissues and supply and drain blood in the heart.
Hence, coronry sulcus separates atria and ventricles externally.