The proximal end of the filament of stamen is attached to the thalamus or petal. Let's understand this in detail:

1. Structure of a Stamen:
A stamen is the male reproductive organ of a flower, which consists of two main parts: the filament and the anther. The filament is a long, slender stalk that supports the anther, which contains pollen sacs. The anther is responsible for producing and releasing pollen.

2. Attachment of Filament:
The proximal end of the filament is the end that is closest to the center of the flower. This end is attached to the thalamus or petal of the flower.

3. Thalamus or Petal:
The thalamus is the receptacle or base of the flower where all the floral organs are attached. It provides support and nourishment to the other parts of the flower. In some flowers, the proximal end of the filament is directly attached to the thalamus.

4. Alternative Attachment to Petal:
In certain flowers, the proximal end of the filament may be attached to the petal instead of the thalamus. Petals are the colorful, leaf-like structures that surround the reproductive organs of a flower. This attachment to the petal provides stability and support to the stamen.

5. Importance of Attachment:
The attachment of the proximal end of the filament to the thalamus or petal is crucial for the proper functioning of the stamen. This attachment ensures that the anther is held in the correct position to facilitate pollination. It also allows for the transfer of pollen from the anther to the stigma of the flower, which is essential for fertilization and the production of seeds.

In conclusion, the proximal end of the filament of the stamen is attached to the thalamus or petal of a flower. This attachment is vital for the proper functioning of the stamen and the successful reproduction of the flower.