Xll] LENTICELS 97 



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

 latter cases there are fine intercellular spaces in the peri- 

 derm-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 sepa- 

 rated 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 periderm-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, AilanthuSy 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 peri- 

 w. i. 7 



