Indehiscent fruit do not split open when ripe. Dehiscent fruits
do split open along a built in line of weakness. An example of a
dehiscent fruit is a peanut. An indehiscent fruit would be an
apple.
An achenium is another word for an achene - botanically
speaking, a small, dry, indehiscent fruit containing a single seed,
such as the buttercup.
What is meant by dehiscent?
A bursting open or splitting along natural or sutured lines. You
probably have seen this when some plants' fruiting bodies open and
their seeds are released.