608 SECONDARY GROWTH IN THICKNESS 



pathological developments. Among Conifers resin -ducts often arise 

 secondarily within the medullary spots. 



The present section may conclude with a few remarks with regard 

 to the organisation of the ventilating system in the woody cylinder. 

 Every portion of the conducting parenchyma of the wood is permeated 

 by a continuous reticulum of narrow air-containing intercellular spaces; 

 in the medullary rays, radial air-passages naturally predominate, whereas 

 the xylem-parenchyma is provided with narrow longitudinal passages. 

 In Conifers, air-spaces also occur between the fibrous tracheides ; 

 according to Paissow, these are particularly numerous, and also 

 relatively wide, in the Gupressineae and Aeaucariae. The same 

 author 301 ' has described two features of purely physiological interest in 

 connection with the intercellular spaces of medullary rays. Where, as 

 usually happens, the cells of the medullary rays are thick-walled, 

 they are provided with pits, even on those walls which border 

 upon intercellular spaces (Larix, Qucrcus, Fagus Ulmus, Salix, Populus, 

 Gytisus Laburnum, etc.). It can hardly be doubted that those 

 pits which border directly upon air-spaces, serve for ventilation ; 

 this is, however, the only instance so far definitely recorded, in 

 which pits appear to serve solely for gaseous interchange. The second 

 point of physiological interest as regards the behaviour of the 

 intercellular spaces of the xylem-rays, consists in the fact that these 

 structures extend without a break right across the cambium and 

 thus communicate with the air-spaces of the phloem-rays, and through 

 them with the cortical air-spaces ; since the latter open to the exterior 

 through the lenticels, the ventilating system of the woody cylinder is 

 really in free communication with the outer atmosphere. This fact 

 has been demonstrated experimentally by Klebahn in the following 

 manner. A twig of Berberis, about 1 cm. in length, is deprived of its 

 extra-cambial tissue for a distance of 2 cm. from the lower end ; the 

 peeled surface and the upper cut end are then coated with some 

 air-tight material. If now air is forced in at the lower end of the twig 

 under a pressure of 14 cm. of mercury, it escapes from the lenticels 

 in the unpeeled region. Owing to the smallness of the pressure 

 employed, this effect cannot be attributed to diffusion through the 

 cell-membranes ; the experiment therefore proves that a direct 

 interchange of gases can take place between the outer air and the 

 ventilating system of the woody cylinder. Klebahn has further shown 

 that, where no lenticels are present (eg. in Vitis spp., Lonicera 

 Periclymenum, Clematis Vitalba, Philadelphus coronarius), the inter- 

 cellular spaces of the medullary rays are continued radially right 

 through the periderm and to the surface, so that here also the wood is 

 placed in communication with the external atmosphere. 



