SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. g6l 



containing air between the rounded edges of its suberdus, phellodermal and meriste- 

 matic cells. By means of these spaces, the intercellular air-spaces of the cortical 

 parenchyma are in open communication with the external air at the time of active 

 vegetation, as may be proved experimentally ; during the resting period of vegetation 

 this connection may be interrupted by an integument of ordinary cork consisting of but 

 few layers. 



In the fully developed lenticel minute investigation shows a phellogenetic layer 

 of meristem, connected with that of the adjoining periderm, and the former, in so far 

 as it belongs to the lenticel, either lies in the same surface as the surrounding portion 

 of the layer, or it bulges out towards the inside, or more rarely towards the outside 

 (e.g. old lenticels of Ginkgo). The cells of the lenticel are either approximately 

 similar in form to those of the surrounding periderm, or, not uncommonly they are 

 narrower in the tangential direction. After its first formation the meristematic layer 

 of the lenticels behaves similarly to the rest of the phellogenetic meristem as regards 

 its production of tissue. Like the latter it always forms phelloderm internally, and 

 in fact in abundance ; the latter may be as much as forty cells in thickness in each radial 

 row, e. g. in old lenticels of Ginkgo. On the outer surface the elements termed by Stahl 

 the complementary cells of the lenticel are first and principally formed in the same manner 

 as cork-cells, and like the latter are arranged in radial rows ; they are approximately 

 isodiametric cells, similar to the cork-cells in form, but varying according to the species, 

 with a thin colourless membrane, which for a long time shows the cellulose-reaction, 

 and only at a later time becomes brown (suberized T), and does not otherwise present 

 any peculiarities of structure ; they have a persistent, colourless, protoplasmic layer 

 lining the wall, and this layer contains a nucleus and often small quantities of starch, 

 and encloses cell-sap which is also colourless. A remarkable peculiarity of the com- 

 plementary cells, especially of the younger ones, which has not been sufficiently investi- 

 gated, is their hygroscopicity, if the expression be allowed, i. e. their tendency to take 

 up water and thus to swell. - The often conspicuous, puffy swelling of the lenticels 

 on the living tree in wet weather depends" on this property, and it is further known \ 

 that after dipping in water the younger internal tissue swells up to a white mass, 

 which exudes from the bursting surrounding tissue, becomes irregularly split up into 

 tatters and fragments, and finally breaks up- on the surface into its separate rounded 

 cells. During this process a considerable, permanent increase in size of the cells 

 takes place, at least in many cases, especially in the radial direction ; e. g. the 

 roundish isodiametric complementary cells of Sambucus nigra become extended into 

 radially placed cylinders, which may become four times as long as broad. 



<n the cases of the formation of lenticels below stomata, to be more exactly 

 described below, the first formed, most superficial complementary cells, diff'er from 

 those described in the fact that they are arranged irregularly, and not in radial rows. 



The complementary cells, like the phelloderm belonging to the lenticel, are 

 rounded at their radial angles, and between the latter interstices containing air exist, 

 which effect the above-mentioned communication of the cortical intercellular spaces 

 with the surrounding air. The rounding is either confined to the narrow corners, 



' De CandoUe, Ann. Sci. Nat. 1826, VII. p. 5. — Von Mohl, Flora, 1832, Verm. Schriften, p. 229. 

 — Unger, Flora, 1836, p. 577, &c. 



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