If you can see thorns on the tree, examine them carefully. It is a quick way of identifying one of the six thorny trees in the guide. Thorns are usually easy to see in the winter when the leaves have fallen but can be used all the year round. Botanically there are thorns and spines. They differ in the way they are formed. Thorns develop from the stem and spines develop from leaf parts. The first three photos show the thorns of the Hawthorn and the Blackthorn and the last photo shows the spine of a Berberis shrub. Other trees that have  thorns are the Cherry Plum, Crab Apple and  Purging Buckthorn but these thorns are on the trunk of the tree and difficult to find. Some False Acacias have spines but often a spineless variety is chosen for city streets. 

The thorn of the Common Hawthorn develops in the axil between the stem and the leaf and it is always above the leaf. Once the thorn has started to grow it cannot develop leaves or flowers

The thorn of the Blackthorn develops at the end of a branch which can develop  flowers.

The spine of a Berberis is a modified leaf  and always appears below the leaf which has been formed from the axillary bud.