Third set of leaves on WW

Question: once the plant breaks the ground the first sets of leaves has a smooth look on top. What is the difference between those leaves and the next set of leaves that has ridges on top? Thanks.

The first set of leafs are called the cotyledon, this set of leafs has stored nutrients that the seedling uses while it is young to keep itself healthy until the root system is big enough to start absorbing nutrients from the medium it is planted in.
The true sets of leafs start to emerge once the roots are established which symbolizes that the vegetative stage of growth has started. The true cannabis leafs are used to absorb carbon dioxide and to convert sugars to energy the plants uses to grow.
(Someone will correct me Iā€™m sure if I have it wrong.)

Here is an article by Robert Bergman which goes through the stages of growth the cannabis plant goes through and explains a little bit about it.