CHAPTER II.— -PLANT TISSUES. 



159 



cellular character, and have their walls thickened by secondary 

 deposit. They sometimes contain starch and traces of proteid 

 matter, but take no active part in the nutritive processes of the 

 plant. They serve it mainly for strengthening or support, and 

 hence have been called mechanical tissues. They are service- 

 able also in conducting the sap. The elements of these tissues 

 are for the most part elongated and oblique-ended or taper 



Fig. 412. 



Fig. 413. 



Fig. 411. 



Fig. 411. — Portion of transverse section through woody part of stem of Pilocarpus 

 pennatifolius. m, m, meduilary ray cells; 7v, iv, wood cells; d, a duct. Magnified 385 

 diameters. 



Fig. 412. — Simple wood cells, isolated, and magnified about 200 diameters. 



Fig. 413. — Wood cell with septum near the middle. Magnified about 200 diameters. 



pointed, though exceptions to the rule are not wanting. Among 

 the shorter forms transitions occur between them and sclerotic 

 parenchyma, and between the fibrous forms and collenchyma 

 every gradation may also be observed. 



(7). Wood or Libriform Cells. These constitute the great 

 bulk of the wood of most plants. They abound particularly in 

 the stems of Dicotyledons, in the area between the pith and the 

 bark. The cells are compactly arranged, long-fusiform in shape, 

 rarely forked or lobed at one or both ends, more or less com- 

 pressed laterally by mutual pressure, so as to appear angular in 



