CHAPTER XXVIII 

 TISSUES AND PROCESSES OF STEMS 



Some of the general regions of stems discussed in the preceding chapter 

 are composed of several kinds of tissues. This is particularly true of the 

 cortex, the bark, and the stele. On the other hand, the pith, vascular 

 cambium, endodermis, cork cambium, cork, and the epidermis each 

 consist of a single tissue. Moreover, some of the tissues of stems are 

 complex tissues, being composed of a system of several kinds of cells. 

 This is particularly true of xylem and phloem, if we regard each of these 

 as but one tissue. 



Stem tissues that develop from cells in the apical meristem are called 

 primary tissues. Those that develop from cells in the vascular cambium, 

 or cork cambium, are called secondary tissues. In stems of most mono- 

 cots, therefore, all the tissues are primary tissues. In stems of w^oody 

 plants nearly all the xylem and phloem and the cork are secondary 

 tissues; and the others, together with the first strands of xylem and 

 phloem adjacent to the pith, are primary tissues. The only primary 

 tissues that may still be present in the trunk of a large tree are the pith 

 and a small amount of primary xylem. On the other hand, stems of 

 herbaceous plants such as coleus consist mostly of primary tissues. Cer- 

 tain microscopic structures in these tissues will be given further attention. 



Apical growth and the origin of primary tissues. At the extreme apex 

 of the stem there is a group of small, closely packed, isodiametric, 

 meristematic cells, the apical meristem. Immediately below it is a zone 

 of cells in various stages of enlargement, some of which are also dividing. 

 In the apical meristem food is being changed into new protoplasm, pectic 

 compounds, and cellulose, as the cells divide and the daughter cells 

 become as large as the mother cells. In the zone characterized by cell 

 enlargement, growth of the cell wall, vacuolation, and the osmotic 

 absorption of water are prominent processes. Apparently there is little, 

 if any, increase in protoplasm, and the protoplasm that is present is 

 being pushed out against the cell wall by the enlargement of the vacuole 

 formed by the coalescence of many smaller vacuoles accompanying the 



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