CHAPTER ELEVEN 



THE STRUCTURES AND PROCESSES OF STEMS 



It 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 cells divide to make other 

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

 this way the growing point is pushed forward and the di- 

 ameter of the stem increased. Upon these two processes, cell 

 division and cell enlargement, the growth of the stem depends. 

 Then certain groups of cells begin to take on special forms. 

 The cells that are to form the bundles elongate ; some of them 

 develop woody walls. Others elongate but remain thin- walled, 

 and these form the food-conducting tissue. The other tis- 

 sues of the stem are composed of cells which have enlarged 

 and have become rounded or variously angled, 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 arise from 

 the small uniform cells of the growing point. 



Stem structures and plant groups. There are three groups 

 of seed plants that we wish to distinguish at this time because 

 the stems of the plants that belong to these groups differ 

 fundamentally. These groups are : (i) the conifers, or cone- 

 bearuig trees, Hke pines, spruces, firs, and cedars, that have 

 scale or needle leaves and are for the most part evergreen; 

 (2) the monocotyledonous plants (monocots), or plants with 

 parallel-veined leaves, like the grasses, Hlies, caimas, orchids, 

 and palms; and (3) dicotyledonous plants (dicots), or plants 

 with net-veined leaves, like oaks, maples, sunflowers, asters, 

 and clovers. 



The stems of the plants belonging to these three groups 



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