ANATOMY OF STEMS 217 



evaporation into the intercellular spaces. The movement of water through 

 the cells of the root and leaf mesophyll must be regarded as integral parts of 

 the process of translocation of water. 



While the great bulk of the water which passes through the plant follows 

 the route just described, and is lost in the transpirational process, small quan- 

 tities escape this fate. All along the route of its movement small amounts of 

 water pass into adjacent living cells and are utilized in cell enlargement, es- 

 pecially in the cambium layer. Actively growing stem or root tips, fruits, 

 etc. also utilize considerable quantities of water principally in the enlarge- 

 ment phase of growth, while chlorenchyma cells utilize water in the photo- 

 synthetic process. In most species, however, not more than i or 2 per cent 

 of the water which enters a plant is utilized in growth and metabolic processes, 

 the remainder being lost from the plant in transpiration. 



That the xylem is the water-conducting tissue of plants has been rec- 

 ognized at least since the time of the girdling experiments of Malpighi in 

 1 67 1. Girdling (or "ringing") a stem, so that all of the tissues external to 

 the xylem are removed, does not prevent movement of water to organs at- 

 tached to that stem above the ring. On the contrary cutting through the 

 xylem tissue of a stem results in almost immediate wilting of leaves attached 

 to the stem above the ring. 



Anatomy of Stems. — Since the mechanism of the movement of water 

 can scarcely be understood intelligently without some knowledge of the 

 anatomy of the tissues through which it moves, a brief review of stem struc- 

 ture is desirable before proceeding further with a discussion of this process. 

 Stems vary greatly in their structure, every species possessing some anatomical 

 features which are peculiar to itself. Nevertheless certain general patterns 

 of tissue arrangement have been found to prevail, and the stem structure of 

 most species will approximate one or another of these general arrangements. 



The stem anatomy of two representative species is shown in Fig. 53 and 

 Fig. 54, which are self-explanatory. The corn stem represents an herbaceous 

 monocot tj^pe of structure while that of the tulip poplar is typical of woody 

 dicot stems. 



The structure of woody stems cannot properly be appreciated merely 

 from a consideration of one year old stems. The primary tissues of such 

 stems develop from tissues formed at the apical growing tip during growth 

 of the stem in length. Nearly all perennial stems also grow in diameter as a 

 result of the development of secondary tissues from the cambium. By successive 

 stages of division, enlargement, and differentiation of cambium cells additional 

 layers of xylem (secondary xylem) are laid down on the inner face of the 

 cambium, and new layers of phloem tissue (secondary phloe?n) on its outer 



