The components of leaves in flowering plants (angiosperms) include the blade, the petiole, and the stipules. There are three tissues found in leaves: the epidermis, the mesophyll, as well as vascular tissue. Each tissue type is composed of layers of cells. The main function of a leaf is to carry out photosynthesis, which provides the plant with the food it needs to survive. Plants provide food for all life on the planet. In plants, the stomata are actually the pores created by the swelling of guard cells to allow CO2 to enter into the leaf, which is a necessary reactant of photosynthesis. The water vapour and O2 are also allowed to escape via the pore. In order to form a pore or stoma, osmotic pressure draws water to increase the volume of the cell; this, in turn, causes the guard cells to bow apart from each other because the inner wall of the pore is more rigid than the wall on the opposite side of the cell.