48 



PEACTICAL BOTANY 



Two other kinds of cells which are important in giving 

 strength to the plant body are shown in Fig. 33. Tracheids 

 (A) form a considerable part of the fibro- 

 vascular bundles and of the wood cylinders 

 of dicotyledons. The slender thick-walled 

 ones, known as fiber tracheids (^?), greatly 

 strengthen those parts of the plant in which 

 they occur. There is considerable difference 

 between wood fibers proper and tracheids in 

 regard to the tissues with which they are 

 generally associated, but the fibers do not 

 differ much in form from the most slender 

 tracheids. Generally, however, tracheids are 

 a good deal stouter than wood fibers, they 

 have rounded ends, and they show spiral or 

 ladder-like markings or oval or nearly circular 

 pits on their sides. Both wood fibers and thick- 

 ened tracheids serve to stiffen and toughen 

 the parts of the plants in which they occur. 



46. Mechanical arrangement of strength- 

 ening materials. Most people know, in a 

 general way, that a metal tube is stronger 

 than a solid rod of the metal, of the same 

 weight per foot of length. So, too, a T-rail 

 for steam or street cars, or an I-shaped girder 

 for bridge or other construction, is much 

 stiff er than one of simple rectangular section 

 like an ordinary plank. 



In many plants the stems and leaves show 

 great economy of stiffening material, having 

 the portions of rigid tissue so disposed as to 

 act in the most advantageous way. Many 

 dicotyledons and great numbers of mono- 

 cotyledons, especially the grasses, have hol- 

 low, nearly cylindrical stems. Sometimes the 

 main stem is not entirely hollow, but portions 



FIG. 33. ^tracheid; 



B, fiber tracheid of 



oak wood 



Magnified about 125 



diameters. After G. 



Mtiller 



