请教 html table表格合并 如何做

请问 html table表格合并 怎么做？
比如
    <table   id= "table1 "   border= "1 "   bordercolor= "black ">
        <tr> <td   bgcolor= 'green '   height= "50px "   style= "width:   50px; "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> </tr>
        <tr> <td   bgcolor= 'green '   height= "50px "   style= "width:   50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> </tr>
        <tr> <td   bgcolor= 'green '   height= "50px "   style= "width:   50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> </tr>
        <tr> <td   bgcolor= 'green '   height= "50px "   style= "width:   50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> <td   width= "50px "   bgcolor= 'green '   height= "50px "> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;   </td> </tr>