veins and capillaries
the veins in a leaf are like three of our body systems in one: they are like the skeleton because they hold the whole leaf open and allow it to capture sunlight and not get damaged too easily by the elements. they are like the circulatory system because distribute water from the roots up to all the cells within the leaf. and they are like a nervous system because there are chemical signals that are transmitted to the leaves from other parts of the plant through the liquid in the veins. they are also hauntingly pretty when all the chlorophyll has retreated and left behind nothing but a pattern of veins.