It's generally a fool's errand to try putting human labels on animal behavior, but it can be possible to define how "good" animals are in terms of their capacity to act selflessly to help others. This is referred to as altruism, and it turns out plants are capable of it as well.
Of course, since this is planets we're talking about, we're not looking at behaviors as complex as of altruistic animals, who have been known to feed, protect, and even raise animals of entirely different species — think of all those stories of primates adopting kittens, for instance. Plant altruism is a little more calculated, and plants likely reserve their self-sacrifice exclusively for members of their own species.
That, at least, is the finding of University of Colorado professor Pamela Diggle, who examined the two "offspring" found inside fertilized seeds of corn. The first offspring is the embryo, while the second is tissue known as endosperm, which is meant to serve as a source of nutrients for the growing embryo, even at the cost of its own existence. Assuming the endosperm isn't fully absorbed by the embryo during the reproductive process, it often becomes the fruit or other parts of the plant that we humans end up consuming.
Most of the time, when a seed becomes fertilized after cross-pollination with another plant, the embryo and endosperm will have the same "mother" and "father." But on certain rare occasions, the embryo and the endosperm will share a mother but have different fathers. It was these cases that Professor Diggle and her research team focused on, to see whether endosperm would be less likely to sacrifice its own existence just to provide nutrients for a relatively unrelated embryo. In a statement, Professor Diggle explains what she found:
"The results indicated embryos with the same mother and father as the endosperm in their seed weighed significantly more than embryos with the same mother but a different father. We found that endosperm that does not share the same father as the embryo does not hand over as much food — it appears to be acting less cooperatively. Our study is the first to specifically test the idea of cooperation among siblings in plants."
While that might indicate endosperm can be selfish bastards, that isn't really news — evolution tends to favor organisms that are selfish bastards, since it increases the chances of surviving to reproduction age. What's more interesting is that, according to these results, endosperm don't simply give up their chance at life just as a matter of course; instead, they actively choose to die if it means their sibling can survive, and they won't do it for just any other plant. That's pretty much the definition of altruism, as co-author and Harvard professor William "Ned" Friedman explains:
"One of the most fundamental laws of nature is that if you are going to be an altruist, give it up to your closest relatives," said Friedman. "Altruism only evolves if the benefactor is a close relative of the beneficiary. When the endosperm gives all of its food to the embryo and then dies, it doesn't get more altruistic than that."