The most common and most likely answer to this crossword clue is the 9 letter word COTYLEDON
I've seen this crossword clue in Independent
C
O
T
Y
L
E
D
O
N
Buddy explains!

The straight part
We know that "plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plant = cotyledon.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does COTYLEDON mean?
COTYLEDON noun- Each of the patches of vili on the foetal chorion in the placenta of ruminants and some other mammals.
- The leaf of the embryo of a seed-bearing plant; after germination it becomes the first leaves of the seedling.