Re: Where do cacti perform photosynthesis?

Cacti perform photosynthesis in the same place as all other plants, inside 
cellular organelles called chloroplasts.  What you are really asking is 
where are chloroplasts located in "leafless" plants like cacti?  The most 
familiar cacti are stem succulents, one of many adaptations plants have to 
very dry environments.  Stem succulents have thick water storing stems. 
Not all cacti are succulents, and not all succulents are cacti, but this 
is an answer to another common question. To limit water loss many desert 
adapted plants have very small or vestigial leaves. This presents a trade-
off problem because reducing water loss with smaller leaves also reduces 
the number of chloroplast containing cells in the leaves. To compensate, 
the photosynthesis function is transfered to cortical cells in the fleshy 
stem, and so their green color. This is not so unusual because many plants 
have green stems when they are young. Photosynthetic stems must also have 
stomates, pores, through which gas exchange takes place.  Again there is a 
functional tradeoff problem because while gas exchange is necessary for 
photosynthesis, water is lost through the pores.  Cacti are among a group 
of plants that solve this problem by capturing solar energy by day, and 
finishing the photosynthesis by night when the stomates can be open with 
minimal water loss.  

Succulent cacti have leaves that are reduced to spines serving to protect 
the water-storing stem.  Prickly pears have very small, fleshy vestigial 
leaves that appear for a short time at the ends of their rather leaf-like 
stems called "pads".  One group of cacti are actually leafy shrubs.  A few 
cacti have adapted to wet tropical habitats where water loss is not such a 
limiting factor, and many of these cacti have broad, flat, thin, leaf-like 
stems.  Some members of this group are sold as houseplants and called 
Christmas cactus or Easter cactus depending on when they tend to flower.  

For future reference, where ever you observe chlorophyll pigmented tissues 
in plants, photosynthesis is taking place.  However, in some cases other 
pigments can hide the green color, for example, red cabbage.  Boiling a 
red cabbage leaf releases the water soluble pigment unmasking the 
chlorophyll green.