Question

why is onion bulb wholly not a true stem

Answer 1

Onion is neither a stem nor a root rather it is a modified leaf. It is a bulb which is considered as an underground stem. It is a modification of stem where the leaves are modified into fleshy scales for storage.

Answer 2

Onion bulb is not wholly true stem:

The onion bulb wholly is not a true stem because, a true stem must possess nodes and internodes which will facilitate the growth of flower and leaves and in turn helps in transportation of food and water across the plants. In case of onion, the bulb of the onion is a modified organ which is involved in storage of food during unfavorable condition. It doesn’t possess any nodes or internodes.