MORPHOLOGY OF HIGHER PLANTS. 347 



the form of a ring known as interfascicular cambium ; xylem com 

 posed of tracheae, wood parenchyma, usually wood libers and 

 sometimes tracheids ; medullary rays separating the collateral 

 bundles ; pith composed of parenchyma and sometimes having 

 stone cells and secretory cells similar to those found in the cortex. 

 In stems having bicollateral bundles, strands of phloem also occur 

 on the inner surface of the xylem rays. 



Importance OF the Study of the Structure of Wood. — 

 The structure of stems in woody plants is very characteristic, not 

 only in different families but in genera and at times in species. 

 In the Pinacese, for instance, the wood is entirely made up of 

 tracheids, the tracheae being wanting. In most of the Dicotyledons 

 tracheae are present, being absent only in certain water plants as 

 Nymphaeaceae and in Drimys, a genus of the ^lagnoliaceae. In 

 the Cactaceae the secondary wood is provided with annular and 

 spiral tracheids. In practical work, whether it be in the study of 

 plants for taxonomic purposes or for their industrial uses, the 

 following are some of the observations that should be made: 

 I. Structure of the walls and nature of the perforations or mark- 

 ings in the tracheae or vessels. 2. The presence or absence and 

 relative distribution of libriform or wood fibers (often referred 

 to as wood prosenchyma). 3. The structure of the medullary 

 rays not only as regards thickening of the walls and contents of 

 the cells, but the number of the cells both as to width and height 

 entering into the individual groups of rays. 4. Wood paren- 

 chyma is variously distributed, the cells aggregated about the 

 vessels being distinguished from those that form tangential bands 

 separating the circles of vessels. The distribution of the crystals 

 is also of importance, as they may occur in isolated cells or in 

 superimposed cells adjoining the wood fibers and form the so- 

 called crystal fibers. 



In the study of different commercial woods it is customary 

 in practice to study the coarser features such as can be recog- 

 nized by the help of an ordinary hand lens. In this superficial 

 study many distinctive characteristics can be readily determined, as 

 the nature of the annual rings, the size of the lumina of the ves- 

 sels as well as their abundance and arrangement, etc. (Fig. 200). 



