What I understand about Cactus plant photosynthesis

We know that trees like Mango trees have leaves, but cactus plants don't. Instead, they have spines. So have you ever asked yourself what they use to perform photosynthesis? Since we know it's done with the leaves for the most part.

Actually, the green part of the cactus which is the stem is responsible for photosynthesis. This is clearly unusual in many plants, leaves handle the light absorption part of the plant's life and stems hold the plant upright.

Cacti (plural for cactus for those unaware) have evolved to live in areas where rainwater disappears very quickly. For cacti, having a flat leaf would be too demanding on them because it would increase water loss, which would cause the plant to wilt from heat over time. So that's why the plant has adapted by relying on its stem to perform all the photosynthesis and store water.

As the plant grows and matures, it loses all its little leaves and when the plant begins to bulk up, the stem takes all the responsibilities for the plant's photosynthesis.

Now please understand the difference here, Cactus spines are not performing photosynthesis, you can think of them as modified leaves. Most people assume that the spines provide some form of contribution to the plant's needs in addition to providing a defence mechanism against herbivores but that is really not the case. Yes the spines do keep animals away from the plant and infact it's why I'm careful when I have to touch, but the spines don't provide food.

There are some species of cacti that do have leaves. Pereskia is one example, I wish I had one to take a photo of it and share. A Pereskia plant resembles more of a shrub than a cactus.

The leaves on Pereskia perform the photosynthesis because these plants have not fully evolved into the storing plants like regular cactus. They still depend on the leaves to do some work for them.

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