Thorny budget

Why such heavy protection?
The holly is one of the few flowering plants with leaves that last more than one year – which also means it has green leaves during the winter, when there is very little other green food around for large herbivores to eat. This combination of long-lived leaves and high visibility in a period of hungry herbivories means that new leaves are a long-term investment worth protecting if the environment has been dangerous lately.
 
Protection only up to a certain point
Even when heavily attacked by herbivores, the holly protects itself only up to a certain height. Take a close look at the next tall holly you see. Leaves will only have spines up to a maximum height of a few metres - above this, all leaves are soft and have no spines. It would be a waste to produce spines any higher than that, since no large herbivores can reach any higher than around two metres.
 
Also: Mysteriously missing berries
Many holly plants flower every year but never have any of the characteristic red berries. Why is that? The answer is deceptively simple: The holly is dioecious a fancy botanical term for each plant having either only male or only female flowers. Male flowers only produce pollen, and female flowers are the only ones that, when fertilised, will turn into berries.