MEDULLARY RA VS. 159 



fibres with a sinuous course. The meshes of the network which thus arise, forming 

 longitudinally elongated slits between the bundles named, are filled up by horizon- 

 tall}' elongated parallel rows of cells, running in the radial direction from the interior 

 to the outside ; these are the so-called medullary rays, which, according as they run 

 in the wood or in the secondary cortex, are to be distinguished as xylem-rays (the 

 "silver-grain'' of the carpenter) or cortex-rays (phloem-rays). It is to be noted, 

 however, that each cortex-ray is simply the outer continuation of a xylem-ray, 

 formed by the cambium. This coarser structure of the product of the cambium 

 is to be seen very plainly in the decomposed stems of Brassica (Cabbage), Carica 

 papaya, and other plants, and still more conspicuously in the woody skeletons of 

 various species of Cactus — e. g. Cereus, Opuniia, and others, where the meshes between 

 the wood- and bast-bundles are unusually distinct. In like manner, the broad bands of 

 the somewhat strongly ligniried(an exceptional occurrence) commercial bast of the Lime 

 illustrate what is here said. In these cases the radial tissue of the rays, which fills 

 up the meshes between the wood- and bast-bundles as they undulate tangentially, 

 becomes destroyed by rotting and the action of the weather in general, because it 

 consists of soft non-lignified cells ; in the true woody plants, on the other hand, as 

 may frequently be observed in the decomposed stems of the Red Beech in woods, the 

 substance of the xylem-rays is occasionally more resistent than that of the wood 

 bundles, and while the latter become destroyed by rotting, the former remain behind, 

 constituting to a certain extent a skeleton of radially disposed plates. On a tan- 

 gential section, moreover, as well as on radial split surfaces of ordinary wood, the 

 larger rays are perceived as bands running horizontally, which traverse the woody 

 mass from within outwards ; the smaller and very small ones are only to be seen 

 with the microscope. Moreover only the first few rays, already existing at the 

 beginning of the growth in thickness, run from the pith through the whole thickness 

 of the wood into the cortex : they break up the mass of wood (seen on the transverse 

 section) into a small number of wedge-shaped portions, with the broad side outwards. 

 The rays subsequently developed with the progressive growth in thickness are much 

 more numerous, and the later they arise, the further they are removed from the pith 

 in the wood. They break this up, as seen on the transverse section, into continually 

 finer, radially disposed portions, arranged in a fan-like manner. It is always to be 

 understood, however, that this breaking-up apparent on the transverse section, is only 

 the expression of the longitudinally elongated meshes in the undulating course of 

 the wood ; and that the rays themselves only represent the filling up of these meshes. 

 I may take this opportunity of remarking, as to the principle lying at the base of all 

 the tissue formation of the higher plants, that, apart from certain organs of secretion 

 and isolated idioblasts lying in the tissue, similar tissue-elements of a plant stand 

 everywhere in contact with one another. As the epidermis, the vascular bundles, and 

 the fundamental tissue, as well as the individual cells of the two latter are all in con- 

 tinuous connection throughout the entire plant ; so with the elements of the wood 

 and cortex, now to be described more in detail, so far as concerns tissue-elements 

 of the same kind. This principle comes out particularly clearly in the case of the 

 medullary rays, inasmuch as these run as parenchymatous nourishing tissue con- 

 tinuously Irom the w^ood into the cortex, and in their turn are connected v/ith the 

 parenchymatous elements of the wood-bundles as well as with those of the cortex. 



