A flower spike is a kind of inflorescence where a group of stalk-less flowers grow on an upright stem. The inflorescence of a plant is the arrangement of the individual flowers on the stem. The spike is categorized as a type of racemose inflorescence, where the stalk-less, or sessile, individual flowers develop around a central flower stalk called the rachis. Lavender and corn are common examples of flower spikes.
Flowers, which are the reproductive organs of the plants, are produced from buds on the axils of the leaves. These flower buds generally have an assemblage of leaf-like structures, which might later develop into secondary buds giving rise to new flowers. The rudimentary leaves of the flower bud are arranged around a central point, which grows to form the stalk or peduncle. In the case of single flowers like the tulip, the secondary buds do not develop and a single solitary flower is borne at the tip of the central stalk.
The flowers where the secondary buds also develop to form a flower cluster are called compound flowers. When the secondary buds of the axis develop into individual flowers with stalks attached to the central axis, it is called a racemeose inflorescence or raceme. The butterfly bush and snapdragon are examples of racemes.
A flower spike is a type of racemose inflorescence where the individual flowers do not have stalks, and are attached directly to the central axis. The spike has several modifications, depending on the arrangement of individual florets on the flower spike stem. Flowers like the anthurium and arum have a type of flower spike called a spadix. The stalk-less individual flowers are arranged compactly on a fleshy rachis, and surrounded by a colorful sheath called the spathe.
Another common modification of the spike is the catkin. They are cylindrical flower spikes which droop down. Common examples of plants with catkin flower spikes are the willow, oak, and birch. In hazel and oak, the female flowers are single; only the male flowers form catkins in these plants.
The sunflower is another modification of a flower spike. Sunflowers, which most people think of as a single flower, are in fact a compound flower. The individual florets are arranged as a compact spike called the capitulum or "head," and is surrounded by colorful leaflets or bracts which we call the petals. It is a typical example for a modification of the spike called involucrum. Daisy, thistle, and artichoke are some other examples of this type.