80 MOUPHOLOGV OF THE CELL. 



257. The drawings of ■wood-elements represented in Figs. 58 

 and 59 are from Sanio's worls, and are given witti his nomen- 

 clature. The cells figured in Nos. 10 and 16, termed by Sanio 

 substitute fibres (German, Ersatzfasern), answer well to the t3-pe 

 of prosenchj-ma. When tliese cells are much reduced in calibre, 

 thej- are known as libriform fibres. 



258. Ordinarj- prosenclyma cells usually have simple pits, but 

 no true spirals. The pits may be round, and of the same size as 

 those on the ducts with wliicli thej- maj' be in contact, but some- 

 times they are elongated slits, and run obliquel}-, as shown in 

 Fig. 59. If two of these cells are in contact, processes may 

 extend from one cell to corresponding protrusions in the other, 

 and thus one cell is united with the next. Hy careful raacera- 

 cion such cells can be sepai'ated, and then each appears to have 

 one or more rows of squai-e teeth or short tubes. It sometimes 

 happens that the wall at the end of these intrusive tubes is 

 broken down, thus allowing free communication between the 

 cells. 



Good examples of substitution cells are to be found in the 

 wood of Magnolia, Liriodendron, man}' Leguminosa;, etc. They 

 are not so common, iiowever, as conjugate parenchyma cells (see 

 Fig. 58). 



259. Woody fibres are of two chief classes : (1) those in 

 which the narrowed cavity is continuous throughout the whole 

 length, and (2) those which have partitions dividing it (sep- 

 tate fibres). 



The flist class has been again divided into two groups depend- 

 ing upon the presence of starch, but the division is not wholly 

 satisfiictory. The first group comprises all those fibres which 

 have a trace of protoplasm, wiiile those of the second have also 

 more or less starch, and generally- some tannin. 



All of these woody fibres resemble the bast-fibres of the inner 

 bark ot dicotyledons so closely that they have been well called 

 libriform. Tliey are described by Sanio, from whose paper on 

 the subject most of these names arc taken, as being always 

 spindle or fibre-form, relatively strongly thickened, and occa- 

 sionally furnished with bordered pits which somewliat resemble 

 those of vasiform elements (264), but are smaller and less 

 clearly defined. Thej- never have true spiral markings, and 

 very seldom any spiral striation. They contain during the 

 periods of rest of vegetation in winter more or less starch, 

 and perhaps some chlorophyll and tannin, but at other times 

 only air. 



