540 SECONDARV CHANGES. 



fibres — the bast-fibres corresponding to the primary vascular bundles — a one- or 

 many-layered zone of parenchyma, connecting the former into a closed ring, 

 becomes converted into stone-sclerenchyma. A ring thus arises, composed partly 

 of fibrous bundles or scattered fibres, and partly of short sclerenchyma, which 

 tra\-erses the border-zone indicated, and is either completely closed at an early 

 period (e. g. Cinnamomum zeylanicum and Fagus), or remains locally interrupted 

 by thin-walled parenchyma for a considerable time, even for years (e. g. Betula 

 alba). Examples of this occurrence of the compound sclerenchymatous ring are 

 aflforded, besides the plants mentioned, by Quercus pedunculata, Q. Suber, Carpinus, 

 Corvlus, Fraxinus excelsior, Juglans regia, Gymnocladus canadensis, Koelreuteria, 

 Negundo, Laurus nobilis, species of Cinnamomum, and many others. In Fraxinus 

 excelsior a compound sclerenchymatous ring, similar in its composition to the first, 

 may arise at a later period in the secondary bast. 



In Fagus silvatica, Quercus Suber, &c., the first sclerenchymatous ring soon 

 acquires projections directed towards the larger medullary rays. In the one-year-old 

 shoot of the tree first mentioned, it shows, opposite each large medullary ray, a 

 protrusion of the adjoining portion of the ring, which here consists of stone-elements 

 uniting the fibrous bundles. The protrusion projects inwards like a ridge up to the 

 neighbourhood of the boundary of the cambium. As growth in thickness goes on, these 

 projecting ridges of sclerenchyma are not only persistent, but increase in the radial 

 direction in such a manner, that opposite each medullary ray they penetrate into the 

 wood beyond the cambial boundary, running between the strands of wood and bast. 

 The cambial boundary is therefore deeply and sharply indented at the medullary 

 rays indicated. The elements of the cortical rays produced by the cambium in this 

 indentation immediately become sclerenchymatous, so that a sclerenchymatous ridge 

 is fitted into the depression. In the medullary rays successively developed at a later 

 period the same process takes place, with the sole difference that their sclerenchy- 

 matous ridges do not extend so far as the external ring. If the cortex of the older 

 stems be torn off from the wood at the boundary of the cambium, the hard ridges of 

 the medullary rays stand out on its inner surface like little combs. 



Similar sclerenchymatous ridges, projecting into the medullary rays, occur without 

 original connection with the compound sclerenchymatous ring, in Platanus and 

 Casuarina. They represent the second case of the formation of stone-sclerenchyma 

 in a definite region. 



In the third case, finally, cells take part in the sclerosis, which may be scattered 

 in all parts of the external cortex and secondary bast. They sometimes occur 

 isolated in tissue which remains soft, e. g. in the outer portion of the bast of Punica 

 Granatum, when six years old ; gigantic scattered sclerenchymatous elements (p. 145) 

 in the bast of the root of the same tree ; ' stone-cells ' in the external cortex of the 

 Cinchonce, Simaruba, &c. In other cases they form larger groups, nests, annular 

 segments, &c., inserted in the soft tissue, the number and size of which may increase 

 in the older parts of the cortex to such an extent that they form its principal mass, 

 so that the old cortex has been appropriately termed 'stone-bark' by Hartig. 

 Casuarina, Platanus, species of Quercus, Betula, Fraxinus, and Acer, the White Fir, 

 also ^sculus Hippocastanum, and especially Fagus silvatica, may be mentioned as 

 typical examples of this structure. 



