Linksys Wireless Router :: WRT160N Fragmentation Threshold

I set the Cisco's AP settings with MTU size as 1200 bytes and the Fragmentation threshold to 256 bytes. Set up: I am using a WiFi enabled laptop connected to Cisco AP running an application pumping data of size 1400 bytes continuously to a Wireless node connected to the same Cisco AP over WiFi.

I am monitoring network activity on the same laptop using Wire shark.However,i cannot see 1400 bytes data getting fragmented. What is the concept of fragmentation and MTU size? Also,I would like to know how to change the AP settings so that i can see the data (1400 bytes)getting fragmented to small byte chunks.