Calyx is the first whorl of the flower and is green and leaf like.Generally  calyx  falls off after the  flower turns into fruit but in strawberry and some plants the calyx remains intact and forms a green whorl on the fruit.Such calyx is called persistent calyx.                                          When this persistent calyx of strawberry was observed under the foldscope , reticulate venation showing the veins and veinlets was seen.This observation clearly shows that the persistent calyx  also performs the function of the leaf.