In a message dated 11/20/2001 12:33:14 PM Eastern Standard Time,
[log in to unmask] writes:

> Tubers such as potatoes are swollen underground stems.   Sweet
>  potato is closely related to convolvulus, a notorious weed with
>  difficult to eradicate, highly invasive, underground stems.

??  I am teaching general biology and our curriculum on botany states that
the white potato is indeed a modified stem, but the yam and sweet potatoes
are modified roots (starch storers).  Hope I am not misleading the students
...
Holly