As a plant molecular biologist with quite a few refereed papers on the subject of cellular communication in plants, please allow me to debunk the unsubstantiated mythology described above. It might easily be argued that because plants can't move they need effective chemical defenses and effective detection and communication. This is the case. You may doubt the sensory and integrative abilities of plants, so I invite you to spend a few weeks in my lab and learn the truth. Plants don't have nerves, since they don't share a particularly recent common ancestor with animals.
Plants feel tissue injury and respond quickly, precisely, and with an effective battery of defenses.
Sure, there is interesting evidence about plants reacting to local tissue damage and even sending signalling molecules serving to stimulate certain chemical defenses of nearby plants. Now, dear reader, please be patient with my indulgence to develop a reasoned reply to such assertive and authoritative pronouncements about plant pain. Although the plant pain promoters are fond of reductios, they will not likely appreciate the following extension of their own. Furthermore, rain clouds bear meaningful information about their level of oversaturation in the form of weight relative to volume. Well, then by the same logic we must do the same to clouds.