LENTICELS. 185 



in the older parts of plants, in which cork-formation has begun ; 

 to all intents and purposes they may be looked upon as cork- 

 stomata. For the winter, somewhat more compact and resistant 

 packing cells are formed. Sambucus does not show in winter a 

 specially formed closing layer of narrow cells lying close together ; 

 while it is met with in many other plants, as also are inter- 

 mediate layers, which, formed just like the closing layer, are from 

 time to time interposed between the packing cells during the 

 period of vegetation. The cells of these closing and intermediate 

 layers become corky, but have radial intercellular spaces between 

 them, so that they do not bring about complete closure. In older 

 parts of the stem of Sambucus the periderm has longitudinal 

 clefts. These pass through the lenticels, without, however, 

 injuring them. The lenticels persist even on quite old stems, 

 while the outer layers of periderm between them scale off. 



Structure of the Cork Membrane. It is advisable to study the 

 structure of the walls of cork cells Jn Cytisus Laburnum (the 

 Laburnum), because here they are somewhat remarkably thick- 

 ened. Cross-sections through the cortex of fairly old twigs show 

 the periderm formed of cork cells of one kind only. These cork 

 cells are arranged in regular radial rows. The youngest cork cells 

 are colourless, older yellow, oldest yellow-brown. Those lying at 

 the periphery appear tangentially stretched, often to the oblitera- 

 tion of their cavity. All these cork cells are greatly thickened, 

 especially on their outer side. We can readily distinguish in 

 them, even without the aid of reagents, the delicate middle- 

 lamella separating the cells, a strong, distinctly laminate, sec- 

 ondary thickening layer, and, within this latter, a tertiary 

 thickening layer. Consequently each complete double wall 

 separating two cell-cavities consists of five distinct layers : the 

 middle-lamella, which here represents the primary cell-wall, and 

 is lignified ; the two secondary thickening layers, which alone 

 are corky ; and the two tertiary thickening layers, which often 

 retain their cellulose character, but here, however, are slightly 

 lignified. 



Chemistry of Cork. If n section of Cytisus Laburnum be 

 treated with a freshly-prepared solution of chlorophyll, as con- 

 centrated as possible, allowing this to act on the section for 

 some time in the dark, it will be seen that the corky mem- 

 branes and the cuticle are coloured deep green, while the lignified 

 membrane and the cellulose walls remain uncoloured. This 



