THE LIVING CELL 23 
are starch granules. They have been formed in the chloroplast 
through the agency of chlorophyll and sunlight by the process 
called photosynthesis. ‘This process is discussed more fully in 
another part of the book. 
THE CELL WALL 
Nearly all plant cells possess cell walls. The cell wall is 
composed of non-living substances which are secreted by the 
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Cy IS tt secondary wall—--- |, 
tertiary walk-~--" , 
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f tertiary — * 
Fic. 14.—Primary, secondary and tertiary walls. A, B, longitudinal and 
transverse sections of trachea of Linden (Tilia americana). C, D, longitudinal and 
transverse sections of tracheid of a yew (Taxus brevifolia). (After Eames and Mac- 
Daniels, Introduction to Plant Anatomy; McGraw-Hill Book Co. publishers.) 
protoplasm.. The youngest meristematic cells in the growing 
apical regions of roots, stems, and leaves are separated from each 
other by delicate primary cell walls made up of carbohydrate 
material called pectose. -As these cells grow older each of their 
protoplasts secretes an individual secondary cell wall composed of 
cellulose and some pectose. So what often appears to be a single 
wall separating two adjacent cells consists of the primary wall 
between 2 layers of secondary wall. The primary wall has 
