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MANUAL OF BOTANY 



CHAPTER IV. 



the stbuctuke of the stem. 



Dicotyledonous Type. 



As we have already indicated, the stem shows considerable varia- 

 tion in its structure, dependent chiefly upon the arrangement of 

 the vascular tissue in the stele. In the great majority of forms 



the stem is monostelic, and 

 its tissues are arranged in 

 one of two plans, leading to 

 recognition of the two types 

 especially characteristic of 

 Dicotyledons and Conifers on 

 the one hand, and Monocoty- 

 ledons on the other. 



The dicotyledonous stem 

 in its very young condition 

 shows us in a longitudinal 

 section of its apex a meristem 

 of small cells, usually dis- 

 playing dermatogen, peri- 

 blem, and plerome (fig. 741). 

 A little way behind the apex 

 the plerome can be seen in 

 transverse sections to be 

 marked off more or less dis- 

 tinctly by the innermost layer 

 of the periblem. 



The epidermis consists of 

 a sheet of cells with outicularised external walls, closely attached 

 to each other, and having no intercellular spaces except the 

 stomata, of which it bears a considerable number. The cells 

 are usually empty save for the presence of water, except in 

 aquatic plants, when they contain chloroplasts. These stems 



Firf. 741. Growing point of the stem of 

 a Dicotyledon. After Douliot. d. Der- 

 matogen. pe. Periblem. pi. Plerome. 

 1. Young leaf. The thick lines are exag- 

 gerated to show the limits of the three 

 regions. 



