I30 



General Botany 



Sometimes the cell walls of both parenchyma and prosenchyma 

 become thickened by the deposition of additional layers of cellu- 

 lose, and the cellulose may be hardened by the addition of Jignin. 

 Cells with thick, hard walls are said to be sclerotic (Greek : scleras, 

 hard) . Sclerotic parenchyma and prosenchyma are often grouped 

 together under the term sclerenchyma. Thus the stone cells found 

 in pear fruits, and in the shells of nuts, may be called sclerotic 

 parenchyma. Bast fibers and wood cells are sclerotic prosen- 

 chyma. If we wish merely to call attention to them as strong, 

 hard tissues, we may call them sclerenchyma. When scleren- 

 chyma cells are mature, they are usually devoid of protoplasm 

 and are filled with either air or water. 



Collenchyma. Another kind of tissue widely distributed in 

 plants and closely related to parenchyma is known as collenchyma. 

 This tissue differs from parenchyma in having the corners, or 

 edges, thickened where three or more cells come together. These 

 thickened edges give rigidity to the tissue, and for this reason 

 collenchyma is often placed among the mechanical tissues. 

 Stem structures and plant groups. There are three groups of 



seed plants that we wish to 

 distinguish at this time, be- 

 cause the stems of the plants 

 that belong to these groups 

 differ fundamentally. These 

 groups are: (i) the conifers, 

 or cone-bearing trees, like 

 pines, spruces, firs, and cedars, 

 that have scale or needle 

 leaves and are for the most 

 part evergreen ; (2) themono- 

 cotyledonous plants (mono- 



FiG. 73. Photograph of a cross-section of cots) , OX plants with parallcl- 

 the outer part of calamus rootstock. The .^^^ ^^ y^^^ ^^^ graSSeS, 



tissue formmg the background is collenchyma, ' _ ° 



in which starch accumulates. lifies, cannas, orchids, and 



