SECONDARV THICKENING. NORMAL DICOTYLEDONS. 485 



properties, with the exception of their shape, and the form of their pits, which in 

 many species is not rounded but slit-Hke. Sanio's term owes its origin to the fact 

 that they not only frequently accompany the short-celled parenchyma of the bundle, 

 but in many woods exclusively represent or replace it, as in Viscum, Porlieria, Cara- 

 gana arborescens, and Spiraea salicifolia. 



2. The short parenchymatous cells of the wood present some differences accord- 

 ing as they belong to the ligneous bundles or to the medullary rays. Thus the 

 parenchyma of the bundles, usually called ^ wood parenchyma^ is to be distinguished 

 from ^\t parenchyma of the rays. 



The parenchyma of the bundles has been principally investigated in the case of 

 hard woods, and it is to these that most of the following statements apply. They may, 

 however, with some obvious modifications, be also extended to the soft, fleshy, suc- 

 culent, or at least loose woods, which consist chiefly of parenchyma, as in the stems 

 of Papayaceae, Bombax, and Chirostemon, and in many roots. 



The cells of the typical parenchyma of the bundles arise by predominant trans- 

 verse divisions of the elongated tissue mother-cells in the cambial zone. Accordingly 

 they are arranged in simple, or in some places multiple, longitudinal rows, each of 

 which has a somewhat spindle-shaped form, resembling that of the mother-cell. This 

 grouping comes out most conspicuously when they lie isolated in non-equivalent 

 tissue, less so where they are united to form larger masses. The length of the 

 spindle-shaped groups is usually less than that of the fibrous cells, more rarely, as in 

 Vitis, it is on the average equal to it. 



The form of the individual cells is that of a more or less elongated prism, with 

 horizontal or oblique terminal surfaces ; it is obvious that those which form the ends 

 of a group must further show a conical tapering. Those which border on wide vessels 

 are often flattened in the direction of the circumference of the vessel, and elongated 

 transversely, in consequence of the expansion of the members of the vessel at the 

 expense of their surroundings (p. 470). 



In many cases the cells which surround a group of contiguous vessels on oppo- 

 site sides are connected by means of flady tubular outgrowths of their lateral v/alls, 

 which penetrate between two vessels, and fit on to one another at their ends. The 

 outgrowths are frequently branched, frequently they have blunt ends without fitting 

 on to others. This phenomenon is explained by Sanio \ and no doubt correctly, by 

 the unequal growth and partial displacement of single rows of parenchymatous cells 

 originally lying between the rudiments of the vessels. It occurs in Casuarina, Mela- 

 leuca imbricaia, Platanus occidentalis, Celtis australis, Ficus Sycomorus, Cordia 

 pallida, and especially in Tectona grandis and Avicennia spec. ; and it has also 

 been found by Sanio in the intermediate cells of Porlieria. 



The wall of the parenchymatous cells of the bundles is in the harder woods always 

 provided with roundish or elliptical pits which are not bordered ; they are never slit- 

 shaped, or arranged in regular oblique rows ; on the sides which are in contact with 

 vessels they are usually larger than on the others, though exceptions to this rule occur 

 (Betula alba). The pitting goes all round, even over the transverse walls, and the 

 latter are of equal thickness with the lateral, or the thinner lateral walls — a point of 



' L.c. p. 94, where the further details are to be compared. 



