People like to say ‘blood is thicker than water’. But plants may actually treat ther siblings better than many of us: although lacking in blood, scientists have found that plants not only recognize family, but respect their space.

The first study to discover that plants were able to recognize siblings was conducted in 2007 on the sea rocket, a seashore plant. In the study, conducted by Susan Dudley of McMaster University in Hamilton, Ontario, researchers found that the plants’ roots would not compete with their siblings but instead would ‘play nice’ and share the space. Now, new research published in Communicative & Integrative Biology shows just how plants, lacking vision and smell, recognize which nearby individuals are familial and which are not.

Harsh Bais, assistant professor of plant and soil sciences at the University of Delaware, painstakingly studied the reactions of wild Arabidopsis thaliana, a common flowering plant that has become a favorite for researchers.

But how do you discover how plants communicate? “Plants have no visible sensory markers, and they can’t run away from where they are planted,” Bais says. “It then becomes a search for more complex patterns of recognition.”

Extracting chemical secretions from the roots (known as exudates) of his study plants, Bais systematically exposed seedlings to the secretions of their siblings, of strangers, and even of themselves.

The study found that when individual plants were exposed to the root secretions of strangers they pushed out with greater lateral root formation, in a sense actively competing with the stranger for room. When Bais inhibited the root secretions, however, this aggressive push outward stopped. The method then by which plants recognize siblings, Bais discovered, is through contact with root secretions. Something in these secretions tells the plant whether it is related or not.

Bais also found that plants growing adjacent to strangers are shorter than if they are grown next to siblings, since strangers place excess energy into their roots. Bais said that he observed sibling plants allowing their leaves to touch and intertwine, while unrelated plants will grow rigidly and avoid physical contact.

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