Question
I bought several orchids during christmas, I have noticed that one of them ( a dendrobium corssed witht something) the base where the roots are attached is turning brown and a little shriveled,there are no new roots,all the leaves have dropped off, and there are about 4 new flower shoots emerging from the very tip of the stems, the other orchids are doing fine they have already given new roots and leaves.

It should be noted that these orchids were imported ( to Guyana from Thailand I think) by some one who sells them and I bought them from her right away.

Answer
Marita, thanks for your question.  If you know what the other genus that is crossed with the dendrobium, that would be helpful, or the proper name of the cross.  It is not unusual for dendrobium canes to turn yelow or brown from intense light.  They are fine as long as they remain firm.  If you feel any softness in the cane, it should be  removed and sealed with a sulphur compound as it is a sign that rot is setting in. Most dendrobiums are deciduous or semideciduous when they lose some or all of their leaves and enter a rest period.  During the rest period, they should remain relatively dry.  This is a reason that the canes shrink a bit.  Since you mentioned that there is new root growth or cane growth, I would keep the watering to a minimum until you see this new growth.  Overwatering a dendrobium during its rest period is the main reason for the loss of these plants.