MECHANICAL TISSUES 285 



layers of cellulose. The thickening is most marked at the 

 corners of the cells, and this gives the collenchyma a highly 

 characteristic appearance, the darker cell contents standing 

 out from the glistening white lozenges of cellulose thicken- 

 ing. Here turgor effects are supplemented by the mechanical 

 strength of the thick walls. As the walls are, however, com- 

 posed of cellulose, and parts of the walls are thin, they 

 remain capable of extension, and the tissue can keep pace 

 with the elongation or expansion of a growing region. 



Woody Tissues. — We may distinguish between four 

 types of lignified cell ; they are not sharply marked off, but 

 are united by intermediate forms, and show varieties of 

 type and mode of origin, many of which have received 

 special names (De Bary, 1884 ; Jeffrey, 1917). They have 

 this in common, that the walls are impregnated with the 

 pentosans and aromatic compounds which convert the 

 cellulose into wood, and are also thickened. The com- 

 pletion of the lignification process coincides with the end of 

 growth ; the death and disappearance of the protoplasmic 

 contents follow. In the mature state these cells have great 

 mechanical strength and but little extensibility. 



(a) Wood vessels or tracheae, which are absent from 

 most gymnosperms, are fused rows of wide elongated cells, 

 the walls of which are thickened in characteristic patterns ; 

 the thickened parts increase mechanical strength, the thin 

 bands or pits permit the ready passage of water. The 

 vessels have primarily the function of conducting water, 

 and the thickening of their walls may be regarded more as 

 securing them from collapse under the pressure of neighbour- 

 ing tissues, especially in their young state, and as withstand- 

 ing the tension in the rising column of water, than as 

 increasing the general rigidity of the axis, {b) Tracheids 

 are single elongated cells, usually narrower than vessels, 

 and with more heavily thickened walls. They form the 

 bulk of the wood of conifers, and are present in most 

 angiosperms. They conduct water, they probably act as 

 water stores, and they also serve as mechanical elements. 

 The tracheids are probably the primitive elements of the 



