CHAPTER VIII 

 ANATOMY OF STEMS, ROOTS, AND LEAVES 



WE HAVE learned that stems are composed of strands of 

 woody, fibrous material embedded in a softer substance or pith, 

 and that this entire structure is enclosed in a water-proof sheath 

 of epidermis or bark. We have also learned that the arrange- 

 ment of these strands of fibrous material, which arc known as 

 fibro-vascular bundles, is different in the moiiocotyledonous and 

 dicotyledonous stems. In the former (Fig. 66) they are dis- 

 tributed throughout the stem except in those plants in which 

 the stem is hollow, while in the latter they are arranged in a 

 circle. (Fig. 67.) This difference in the arrangement of the 

 mono- and dicotyledonous stems enables us readily to recognize 

 these two groups of plants. (Chapter III.) 



Cellular Structure. If we can examine cross and longitud- 

 inal sections of a moiiocotyledonous stem with a microscope, we 

 will find that the pithy part of the stem is composed of large, 

 thin-walled cells. (Fig. 68.) We have previously referred to 

 cells, but have not given an explanation of them. All parts of all 

 plants are made up of cells of which this is the simplest type. 

 The cell is the unit of the plant structure, the same as a brick 

 or a stone may be the unit of a building. The name " cell " 

 was given by the students of botany, soon after the invention 

 of the microscope, who saw the plant as a structure composed 

 of many minute, apparently empty boxes or cells. Later 

 students learned that these cells when young and active con- 

 tained a substance which we now call protoplasm, (Fig. 60), 

 and still later students learned that protoplasm might exist with- 

 out being enclosed in a cell-wall. Therefore, the definition of 



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