CHAPTER SIXTEEN 



THE STRUCTURE OF STEMS 



If we study the development of a stem from a bud, we find 

 that the growing point is made up of very minute cells, all of 

 which are practically alike. These cehs divide, making other 

 cells like themselves, and the lower ones begin to enlarge. In 

 this way the growing point is pushed forward and the diameter 

 of the stem increased. Then certain groups of cells begin to take 

 on special forms. The cells that are to form the bundles elongate 

 and the cross-walls between some of the cells disappear, forming 

 the water -conducting tissue. Some of the cells develop thick 

 woody walls {wood tissue). Others elongate but remain thin- 

 walled, and these form Xh^ food-conducting tissue. Just outside 

 the food-conducting tissue very slender elongated cells with 

 thick walls develop (the hast). The other tissues of the stem are 

 composed of cells which have enlarged and have become rounded 

 or variously angled through mutual pressure and which have their 

 walls more or less thickened. These cells form the pith or soft 

 inner part of the stem, and the cortex or outer portion. In this 

 way the various tissues of stems composed of a variety of cells 

 arise from the small uniform cells of the growing point. 



This tissue composing the growing points of leaves, stems, and 

 other organs is called meristematic tissue, or simply meristem. 

 The tissues that are formed from the meristem are composed of a 

 variety of cells, which have been classified into several types to 

 facihtate the description of plant organs. 



Parenchyma and prosenchyma. The cells that make up the 

 epidermis and mesophyll of leaves, the softer parts of herbaceous 

 stems, and the fleshy fruits are either spherical or spheres that 

 have been compressed. Their length and breadth are not very 

 different and they are arranged in rows and layers, with their walls 

 touching each other. Cells of this type make up parenchyma 

 tissue. In the veins of leaves and the bundles of stems there are 



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