122 LENTICELS [CH. 



In some cases it is the dwarf-shoots which harden off 

 to a sharp point and form the thorns, e.g. in 



Pear Hawthorn Buckthorn 



Blackthorn Sea Buckthorn. 



Their true nature is easily detected in such cases by 

 their axillary position or by their bearing buds, &c. 



Before dismissing the subject of leaf-scars, it is neces- 

 sary to remember that in some trees the buds are actually 

 buried in the tissues beneath the scar, and burst forth 

 through the surface of the latter. Conspicuous examples 

 are found in Robinia and Philadelphus. These cases are 

 to be kept distinct from those where the scar merely 

 surrounds the bud, as in Rhus typhina and the Plane 

 (Fig. 19), though they are connected by intermediate 

 conditions. 



Most twigs are furnished with small patches of loose- 

 celled tissue, bursting through the epidermis as little 

 heaps, and furnishing passages for the ingress and egress 

 of gases and watery vapour to the tissues of the cortex 

 beneath. These wart-like excrescences are the lenticels, 

 or corky warts, and their formation and structure will be 

 dealt with later on : meanwhile we are concerned rather 

 with their diagnostic value. Lenticels are a prominent 

 feature on the twigs of the following. They are large, or 

 at least conspicuous in 



Alder Walnut Beam 



Poplars Chestnut Elder 



Horse-chestnut Maple Elms 



Robinia Birch Rowan 



