According to various sources on the Web, there are over 300,000 registered species of plants on Earth. Based on that statement alone, we can safely say that through evolution or clever design, their instinct for survival has helped them prosper. Even though you can draw the argument that some of their features resemble human senses, can you really say that plants can actually think for themselves? As we all know, plants are stationary organisms, which you must agree is an overwhelming disadvantage. Even with that, they make up a cumulative mass which is a thousand times higher than that of all animal species. This is where the adaptive skills of plants come into play.
Just like humans, plants have a distinct sense of smell and can react rather quickly to chemical changes in their surroundings. When the fruit starts to ripen and release ethylene, neighboring plants begin to ripen faster. Here is another one: certain plants give off scents to attract insects for the purpose of pollination. Plants like the carrion flowers go to such extremes as to grow tiny hairs, become warmer and emit a smell similar to the smell of rotting flesh in an attempt to draw flies and beetles to expedite the process.
Passive reaction and teamwork
Furthermore, when pests, animals or pathogens attack plants, they react. The acacia tree, when grazed by passing animals, starts producing a chemical compound called Tannin, which makes their leaves extremely hard to digest. Some acacia plants were even known to produce enough of this chemical to actually kill an animal.
Plants are also known to work in symbiosis. Fungi, with their underground webs, can make a pathway between tree roots, which in turn allows the plants to exchange nutrients and information about their surroundings. Larger specimens of trees are known to nurture younger or shaded trees by sharing water and nutrients. This is especially prevalent in the exchange between evergreen and deciduous species. The evergreen species share their nutrients with the plants that lose their leaves during the winter, and this is reciprocated over the summer season.
Active reaction and memory
We left the most shocking examples of sentient behavior in plants for last. The Mimosa Pudica, commonly known as the Shame plant, closes its leaves when it receives an outside stimulus. This was developed to scare away insects that land on them. A scientific experiment was conducted, during which the leaves would be dropped from the height of 15cm and they would close when they hit the ground. After the scientists repeated the experiment for four or five times, some of the plants would stop closing. This effect would last for weeks at a time, proving that plants, in fact, could form concrete memories. This was further affirmed when those same plants, during the period when they would not close when dropped, would close their leaves when being shaken, proving they react differently to a different kind of stimulate.
Now, not to get ahead of ourselves, plants are still missing key parts to be considered sentient beings. They do not have organs a brain or other similar organs deemed necessary by scientists for a creature to be considered sentient. And although they are missing an organ that would serve as a brain, plants still somehow manage to survive being eaten and overcome the fact that they are immobile without much effort. Plant sentience comes down to your definition of intelligence. If teamwork, environmental interaction and strong survival instincts are something you rank highly, then there is a strong case to be made for plants being intelligent. This is something to keep in mind when next time you get into a debate with a vegan about animal cruelty.