80 MORPHOLOGY OF THE CELL. 



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

 and 59 are from Sanio's work, and are given with his nomen- 

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

 substitute fibres (German, Ersatzfasern), answer well to the t^ype 

 of prosenchyma. When these cells are much reduced in calibre, 

 the} T are known as libriform fibres. 



258. Ordinary prosenchyma 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 which the}' may be in contact, but some- 

 times they are elongated slits, and run obliquely, 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. By careful macera- 

 tion such cells can be separated, and then each appears to have 

 one or more rows of square 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}- Leguminosse, etc. They 

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

 Fig. 58). 



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

 which the narrowed cavitv is continuous throughout the whole 



ts 



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

 tate fibres). 



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

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

 satisfactory. The first group comprises all those fibres which 

 have a trace of protoplasm, while 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. They are described by Sanio, from whose paper on 

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

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

 sionally furnished with bordered pits which somewhat resemble 

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

 clearly defined. They 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 

 onlv air. 



