46 COLLEGE BOTANY 



shell-like structure, which is made up of small, often dense, 

 fibro-vascular bundles (Fig. 22). A cross-section of a hollow 

 monocotyledonous stem, such as the wheat, shows the same gen- 

 eral character, except that the bundles are arranged in a circle. 

 The fibro-vascular bundle is made up of the same tissue as the 

 bundle of the dicot stem, except for the meristematic tissue or 

 cambium, which is present in the young but absent in the old 

 parts of the bundles. The growth in a fibro-vascular bundle of 

 this type is restricted to the youngest or terminal part. There- 

 fore, after reaching a certain period in their development, 

 moiiocotyledonous stems cease to increase in diameter and grow 

 in length only. 



The Growth of the Dicotyledonous Stem. The growth of 

 the dicotyledonous stem involves : ( 1 ) cell divisions in the meri- 

 stematic regions resulting in the formation of many new cells ; 

 (2) the enlargement of cells, and (3) the differentiation of the 

 cells into the various tissues to which we have already referred 

 (Chapter II). -The details of this process are as follows: The 

 primordial meristem (Fig. 25) is a mass of undifferentiated 

 cells at the top of a growing shoot They are the actively grow- 

 ing parenchyma cells which give rise to all the other tissues of 

 the stem. The first differentiation of the mass of cells gives 

 rise to three more or less distinct groups of cells just back of the 

 growing point. They are the protoderm, ground meristem, 

 procambium and constitute the complete primary meristem. 

 Each of these three groups of cells gives rise to other groups 

 of cells as follows: 



The protoderm is gradually transformed into the epidermal 

 structures previously referred to. The procambium is gradually 

 transformed into the fibro-vascular bundles, which are composed 



