Plant Anatomy and Physiology of an Onion

Roots

• Some plants, like the carrot, for example, have tap roots featuring one dominant central root that penetrates downward into the soil. Others have a fibrous root system consisting of a fairly dense mass of smaller individual roots. The onion has a fibrous type of root system. Onion roots carry out functions typical of many plants. They have an internal microscopic structure of conducting tissue, termed xylem and phloem, that serves both as structural support and to transport water and nutrients throughout the plant.
  
  

Stem

• The stem is a feature common to many flowering plants. In many plants the stem is an elongated, more or less vertical structure. The onion is a bit unusual in that its stem is considerably reduced. Working down from the above-ground parts of a mature onion plant, an observer doesn't reach the stem until well underground, at the base of the onion's bulb. But despite its different form, the onion stem's physiology and function is like the stems of other plants in that it conducts water and nutrients to other parts of the plant.
  
  

Leaves

• It may come as somewhat of a surprise that the onion's bulb, including those layers that we might slice through when making a salad, are actually leaves. They are highly modified leaves, but still leaves nevertheless. Those modified bulb leaves serve as nutrient storage structures for the plant. At the top of the onion's underground parts -- the roots and bulb -- is a continuation of the onion's set of leaves. The onion's above-ground leaves, though, are tubular and green. Their physiological role is to absorb light energy and carry out photosynthesis. This physiological process manufactures the sugars that nourish the onion plant, including the bulbous leaves below.
  
  

Flowers and Seeds

• Although the gardener who grows onions may not be interested in the onion plant's flowers or seeds, the onion is a flower- and seed-producing plant. It generates flowers that botanists classify as umbels. An umbel is a flower head of multiple individual flowers that resembles an umbrella. Onion flowers, once pollinated, develop and mature into seeds. When the onion's seeds encounter a suitable environment, they germinate, mature, flower and fruit and thereby continue the onion plant's life cycle.