How a marijuana plant works.

LEAVES are where photosynthesis- the process in which sunlight combines with CO2 and water and is then taken up by the roots to manufacture food (carbohydrates) for the plant - takes place and oxygen is released as a by-product. Plants regulate small openings, stomata, on leaf undersides to allow CO2 to enter and oxygen (O2) to exit. When open, stomata allow water vapor to escape in the process of transpiration.
ROOTS anchor a plant in the ground and absorb water, nutrients, and air. Hydroponics allows absorption of water, nutrients, and air at an accelerated rate. Tiny root hairs increase the surface area for absorption. These root hairs are very delicate and must be moist at all times. Large roots are similar to stems. They transport water and dissolved minerals (fertilizer) in the phloem.
Stems have a vascular system to transport water and nutrients throughout the plant. The xylem carries water and dissolved minerals from roots to leaves. The phloem transports food manufactured by leaves to stems and roots. The vascular cambium is the growth zone that produces xylem on one side and phloem on the other. The stem also supports the plant and bears leaves, and buds.