492 SECONDARy CHANGES. 



Schwendener (Mechan. Princip, p. 148, sub. 3), Tropseolum, Impatiens, Centranthus, 

 and Cachrys, perhaps belong to this category, the others not. 



Ill any case all the exceptions of the category last-mentioned constitute transi- 

 tional forms to those mentioned at p. 458, where there is no sharp lateral limitation 

 of distinct ligneous bundles by radial bands of non-equivalent structure, and hence 

 medullary rays cannot be distinguished at all, whether it be that only the large 

 medullary rays are absent and small secondary ones appear later, as in Ephedra and 

 Cobaea, or that rays of every rank are absent throughout, as in Crassulacese, Centra- 

 denia, Rumex Lunaria, and Campanula Vidalii. 



Sect. 148. In many woods, e.g. constantly in species of Alnus and Sorbus, 

 accumulations of parenchymatous cells occur, constituting as it were local hyper- 

 trophies of the medullary rays ; these were first described by T. Hartig as cellular 

 passages, subsequently by Nordlinger as medullary spots, and by RossmSssler as 

 repetitions of the pith. According to the investigations of these authors and of 

 KrausS these structures appear in cross-section in the form of elongated spots, 

 usually at the outer side, but not unfrequently in the middle of an annual ring, with 

 their greatest diameter following the periphery of the ring, while they often form 

 considerable annular segments extending through 90° and more'. In the vertical 

 direction they extend like passages for distances of several feet, sometimes ending 

 blindly, sometimes branched here and there, not unfrequently crossing one another in 

 their irregular course. They are often conspicnous to the naked eye by their brown 

 colouring, e.g. in the trees mentioned ; in other cases they are colourless, e.g. Populus 

 monilifera, and tremula. They consist of irregularly polyhedral, irregularly arranged 

 cells, with thick pitted walls, contents including starch, tannin, &c. ; the latter are generally 

 brown in the dry wood, and chiefly contribute to the colouring of the spots. This may 

 also partly depend on very thin-walled, compressed (partly disorganised ?) cells at the 

 circumference of the spot, as described by Kraus in the case of Sorbus torminaKs. 

 They owe the names given them to the similarity of their thick-walled cells to those 

 of the pith, especially of its periphery. The medullary rays coming from the middle 

 of the stem enter the inner side of the spots, their cells becoming broader as they 

 approach the latter, and assuming more and more the characteristics of those of the 

 spots ; thus the medullary rays pass over from within outwards into the passages ; 

 they further coalesce with them laterally. On the outside new medullary rays start 

 from the passages, and as regards their direction these are either independent of those 

 coming from within, or lie in the same straight line with them. Local swellings of 

 the medullary rays, due to increased breadth and number of their cells, which latter 

 may assume irregular forms, are immediately connected in structure with the smaller 

 spots or passages of this kind ; and this also applies to the longitudinal union of 

 neighbouring rays, by means of small groups of parenchymatous, more or less 

 irregular cells, which abut on them. Such structures occur in Abies alba, balsamea, 

 Pichta, in Cunninghamia, Cupressus sempervirens, and frequently also in Abies pecti- 

 nata. In the Coniferae, and in Liquidambar, hysterogenetic and lysigenetic resin- 



' Hartig, Foistl. Cultnrpflanzen.— Nordlinger, Querschnitte, Bd. II.— Kraus, Nadelholzer, /. <r. 

 p. 162. 



