SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 563 



projecting outgrowths of cork, does the converse case in a certain sense appear, the 

 lenticels lying in the depressions between the wings. In the case of the thick corky 

 layers of Quercus Suber also, the inequalities of which are principally due to 

 mechanical cracking, the lenticels do not appear above the surface ; they traverse 

 the layers in the form of radial, irregularly constricted cylindrical columns, consisting 

 of a loose mass of complementary-cells which become brown on drying, and extend 

 from the withering surface to the phellogen ; they are known to everybody who has 

 seen a cork stopper as the brown pulverulent stripes, running at right angles to the 

 annual layers. 



A lenticel, belonging to a persistent periderm which increases successively 

 in circumference with the growth in thickness of the shoot, behaves differently 

 according to the species, as regards its own growth in breadth. In numerous kinds 

 of trees, as Prunus avium, Betula, Abies pectinata, and Tamarix indica, every lenticel 

 apparently increases in breadth in about the same proportion as the circumference of 

 the shoot. On old stems or branches the lenticels appear as large, transverse 

 segments of rings. Although accurate measurements are wanting it may yet be 

 asserted of these cases with approximate exactness, that the portion of phellogen 

 belonging to the lenticel follows the dilatational growth in the same way as is known 

 to occur in the rest of the periderm, and persistently forms lenticel-tissue. 



In other cases, e. g. Fraxinus excelsior, F. Ornus, Ailantus, and Quercus Suber, 

 the lenticels show little or no increase of breadth, or even show a decrease as they 

 become older. In a third series, finally, e. g. Pyrus JNIalus, Rhamnus Frangula, 

 Broussonetia, and Tsuga canadensis, as well as Quercus Suber, a lenticel may be 

 divided up into several smaller ones by dense periderm. The latter case can only 

 arise owing to the fact that at certain points in the phellogen of the lenticel, cork in- 

 stead of complementary tissue is formed from a definite point of time onwards (no 

 doubt from the date of the autumnal formation of the cork-layer). The same process 

 must go on progressively from the periphery towards the centre of the lenticel, when 

 the latter diminishes in its superficial extent. Where the latter remains the same or 

 increases slightly, it is doubtful whether the process just mentioned takes place in 

 proportion to the dilatation of the phellogen of the lenticel ; or whether, as is less 

 probable, the latter takes no part in the general dilatation of the cortex, or a lesser 

 part than the periderm outside the lenticel. 



According to what has been stated, the absolute size of the lenticel may change 

 considerably with age, in the same individual, the transverse diameter attaining the 

 length of icm and more, in lenticels which participate in the growth for a very long 

 time. The original superficial size, which remains unchanged in those growing but 

 little in breadth, may be stated at about i"!'". In periderms which are quickly cast 

 off, e. g. that of the Plane, considerably smaller lenticels occur, which can scarcely 

 be clearly distinguished with the naked eye. 



The origin of the lenticels shows differences respecting the region and mode of 

 their formation, according to the position of the periderm to which they belong. 



Where the seat of the first formation of periderm is the epidermis, or the sub- 

 epidermal layer of parenchyma, or, as in the above-mentioned Leguminosae, a slightly 

 deeper layer, the lenticels arise delow the slomala, namely one under each stoma when 

 the latter are not very numerous and are uniformly distributed, e. g. Sambucus nigra, 



002 



