XII] LENTICELS 97 



odoratus, and perhaps one or two others; and even in 

 these latter cases there are fine intercellular spaces in 

 the periderm-layers over the ends of the medullary rays, 

 which represent lenticels in function, in that they permit 

 a slow interchange of gases with the exterior. 



For the lenticels are nothing more than spots in 

 the periderm where the cork-cells become loosened and 

 separated sufficiently to allow transpiration and the free 

 passage of air to and from the exterior they replace in 

 fact the stomata of the epidermis, and are frequently 

 formed below them. As a rule they are blocked up 

 during the winter by the most recently formed pefiderm- 

 layer, and are forced open in spring, when the loose 

 powdery cork-cells often protrude so evidently as to give 

 rise to the name cork-warts. 



Excellent diagnostic characteristics are afforded by the 

 abundance, prominence, size and shape e.g. elongated 

 longitudinally or transversely and the colour contrasts 

 of these lenticels on the younger branches, &c. 



The tissues resulting from the activity of the cork- 

 cambium are, with the latter, included in the term 

 periderm, and it is clear from the foregoing account that 

 the periderm may be superficial or deep-seated in origin. 



But in many trees, while the first formed periderm is 

 superficial (except for the rapidly sloughing epidermis) 

 and remains so for years by continued activity of the 

 cork-cambium which keeps pace with the thickening of 

 the branch e.g. Beech, Hornbeam, Hazel, Barberry, 

 Privet, Ailanihus, Birch, &c. matters are complicated in 

 others by the death of the first superficial periderm after 

 the first year, and the repeated formation of more deeply 

 situated new periderms year by year. 



These latter cases, therefore, are after the first year 

 comparable to those mentioned above, where the periderm 



W.I. 



