486 SECONDARy CHANGES. 



distinction from the septate fibrous cells. The walls, though lignified in the mature 

 condition, are uniformly distinguished from those of the tracheae and fibres of the same 

 wood by their lesser thickness. Exceptions to this rule are very rare : Magnolia acu- 

 minata and tripetala, Lirlodendron tulipifera, Gymnocladus canadensis, and Amorpha 

 fruticosa, in which the radial walls of those woody parenchymatous cells which lie in 

 the autumn wood are not inconsiderably thickened. Spiral or annular fibres are 

 always absent. The parenchyma of the bundles in soft fleshy woods is in general 

 only distinguished from that above described, by its usually larger cells, and less 

 thickened walls. 



The nature of the cell-contents is in general characterised by the term paren- 

 chyma. In most hard woods the starch-grains, which are stored up periodically, 

 during the winter's rest, form the chief contents ; chlorophyll and tannin occur here 

 and there ; the former, for example, in the wood of Cobsea scandens. 



The parenchyma of the yneduUary rays consists, in the great majority of secondary 

 woods, of cells which have essentially the same properties as the parenchyma of the 

 bundles in the same plant, without being exactly similar in every point. In woods 

 which are not fleshy and succulent the walls of the cells of the medullary rays are as 

 a rule lignified, like those of the woody parenchyma. Exceptions to this rule occur 

 in many twining and climbing plants, in which the cells of the medullary rays remain 

 unlignified, delicate, and capable of yielding to pressure and tension, e. g. Menisper- 

 mum canadense, Aristolochiae, Atragene alpina. The lignified medullary rays of 

 Clematis Vitalba, which agrees so closely in every respect with Atragene, show how- 

 ever that this phenomenon is by no means generally characteristic of plants with the 

 habit mentioned. 



The form of the cells of the medullary rays is usually that of a rectangular prism, 

 often with rounded corners, and roughly comparable to a brick ; in thin medullary 

 rays, filling up a narrow mesh between the ligneous bundles, the cells which occupy 

 the angles of the mesh have a corresponding wedge-like form. Usually the cells are 

 chiefly elongated in one direction, and are either procumbent with their greatest 

 diameter directed horizontally and radially ; or upright, with their greatest diameter 

 vertical. The former is by far the most frequent case. Cells standing vertically occur, 

 for example, in Asclepiadege (Periploca, Hoya, Asclepias curassavica), Nerium, Drimys 

 Winteri, and Medinilla farinosa. In the medullary rays of Camellia japonica pro- 

 cumbent and verdcal cells occur in groups. Medullary rays with procumbent cells 

 are always easy to distinguish from parenchyma of the bundles, even where they 

 traverse the latter, because the longitudinal diameters of the two kinds of cells cross 

 one another \ in the case of the upright cells this distinction is often less simple on 

 account of the similar direction of the longitudinal diameters. 



But few minute investigations on the structure of the cells of the medullary rays exist, 

 and many details are still to be discovered. F'rom what is already known, it may 

 however be asserted that the cells of a medullary ray are as a rule similar to one 

 another, apart from irrelevant differences, some of which follow directly from what has 

 been said. But few exceptions to this rule are known. In the medullary rays of Aristo- 

 lochia Sipho, Sanio^ found smaller cells, containing finely granular starch, arranged in 



' L.c. p. 127. 



