82 THE BOOK OF NATURE STUDY 



The prickles of the Rose are of the nature of hairs, because 

 they are developed from the epidermis. These are often popularly 

 called thorns, but the term " thorn " in botanical language 

 generally represents a modified branch, as in the spines of the 

 Blackthorn and Hawthorn. Sometimes thorns represent modi- 

 fied leaflets, not branches. The leaves of the Barberry plant 

 show an interesting transition from foliage - leaves to spines. 

 The lowest are foliage-like, the uppermost spiny ; the intermediate 

 ones are smaller than the lowest, and end in a spiny process, 

 so that they are something between the foliaceous and spiny 

 forms. The thorns or spines of the Gorse are either modified 

 branches or modified leaves : those that arise in the angle which 



FIG. 49. Different forms of hairs : Stock, simple, and branched ; 

 Stinging Nettle and Mullein. 



the leaf makes with the stem are from their position branches ; 

 whilst those that are borne laterally by the main stem or branch 

 are modified leaves. The young seedling of the Gorse has tri- 

 foliate, ordinary foliage-leaves (Fig. 22), but as the plant grows 

 older these are replaced by spines. It seems likely that these 

 spines have gradually been developed, possibly in the first instance 

 to prevent too rapid transpiration in dry seasons, the Gorse 

 liking a sandy soil ; now the thorns also serve to protect the 

 plant from herbivorous animals. 



The spines of the Garden Acacia (Robinia) may also be men- 

 tioned. These arise at the base of the leaf-stalk, and are modified 

 stipules. 



