Essential to understanding the life cycle of a plant is the concept of alternation of generations. One of the defining traits of plants is that a single adult plant exists in two separate states, called generations, with differing purposes. The sporophyte generation contains a full set of chromosomes in each cell and produces spores.
SD cards are an extension of MMC memory cards; this means that it's possible to use MMC cards in newer SD devices. It's also possible to use regular SD cards in SDHC and SDXC devices.
SD stands for Secure Digital, SDHC stands for Secure Digital High Capacity, SHXC stands for Secure Digital eXtended Capacity. From the names only you can tell he main difference in all three i.e. storage capacity. SDHC cards offers between 4GB to 32GB and SDXC offers more than 32GB.