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PLANTS AND MAN 



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tirely of tracheids. Since tracheids have relatively thinner walls 

 than the more specialized sclerenchyma cells of dicots, conifer 

 wood has less solid material, volume for volume, than Dicot 

 wood. For this reason conifers are generally known as softwoods; 

 their wood lacks the strength and density of most Dicot woods, on 

 the other hand the rather uniform nature of the tracheids results 

 in a straight-grained wood which is easily worked. 



Collenchyma 



CoLLENCHYMA cclls are the simplest of the specialized sup- 

 porting tissues, being intermediate between ordinary parenchyma 



Fig. 173. — Collenchyma cells 

 are the simplest of the special 

 supporting tissues, being inter- 

 mediate between parenchyma 

 and sclerenchyma. 



Fig. 174. — Stone cells 

 are shorter than tracheids, 

 and have small cavities 

 surrounded by thick ligni- 

 fied walls. 



and sclerenchyma. Collenchyma cells are slightly longer than 

 broad with unevenly thickened cellulose walls, the thickening 

 usually occurring as longitudinal strips at the angles of the cell 

 (fig. 173). They are a temporary supporting tissue found chiefly 

 in herbaceous stems, leaf stalks and veins of leaves where they 

 occur as longitudinal strands. 



Sclerenchyma 



Sclerenchyma tissue is of two types — stone cells and fibers. 

 Stone cells are the least common and least important of scleren 

 chyma tissues. Each stone cell (fig. 174) is shorter than a tracheid 

 and has a much smaller cell cavity and thicker lignified walls 



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