32 THE TISSUES OF PLANTS 



the pith cells of one or two year old twigs of apple. These are 

 also somewhat thick- walled. 



41. Sclerenchyma is the name given to a tissue con- 

 sisting of more or less rounded or polyhedral, usually 

 not much elongated, thick-walled cells whose function is 

 to give support or protection to other tissues. These 

 cells originate from meristem by the thickening and 

 lignification of the walls, passing through an intermediate 

 parenchymatous stage. During the process numerous 



spots on the walls remain thin so that 

 eventually they show as canals from the 

 small central lumen of the cell to the 

 original outer wall. These canals or pits 

 Fig. u.-seierenohyma Correspond in adjacent cells. Upon 

 reaching their final development the cell 

 contents die. Sclerenchyma cells are often called stone 

 cells. They are found in seed coats, nut shells, bark, 

 etc., where protection or support is required. 



42. Of a much different type from the foregoing are 

 those tissues consisting of elongated cells with more or 

 less thickened walls whose function is the mechanical 

 strengthening and support of the plant body. To per- 

 mit bending while at the same time retain- 

 ing their supporting function they are more 

 or less elastic, a characteristic less marked 

 in the short-celled sclerenchyma whose func- 

 tion is protection or only local support. „ 



Ttr T • -1 r 1 ^'°- 12.— Collen- 



We can distinguish two types of these sup- ohyma. 

 porting or mechanical tissues, collenchyma and fibrous 

 tissue. 



43. Collenchyma. Directly beneath the epidermis of 

 many plants are found smaller or larger strands of elon- 

 gated cells whose longitudinal cell walls are thickened at 

 the angles where three or more cells come in contact. 



