SECT. I. ORGANOGRAPHY AND GLOSSOLOGY. 



23 



name of tracheae. The diameters of most ducts are 

 generally larger than those of the true tracheae, be- 

 longing to the same plant ; and the dotted ducts, espe- 

 cially, are very distinctly visible to the naked eye, and 

 even large enough to admit of a delicate hair being thrust 

 into them, where they are divided by a transverse sec- 

 tion of the stem. 



(25.) Woody Fibres and Layers. When a piece of 

 wood is split longitudinally, or in the direction of the 

 stem, it cleaves more readily than when it is broken 

 transversely. And many kinds -of wood may be thus 

 split in the direction of the grain, into very thin 

 layers, and these again be subdivided into fibres of ex- 

 treme tenuity. The fibres obtained by macerating flax, 

 hemp, and other plants used for cordage, are of this 

 description. If these fibres are examined under the 

 microscope, it will be seen that they do not consist of 

 continuous tubes or filaments alone, but are composed 

 of various combinations of vascular and cellular tissue. 

 Every separation in the direction of the fibres (fig. 14. 

 a a) occasions the disunion of a u 



contiguous tubes or vesicles, but 

 any transverse fracture (6 &') 

 can be obtained only by the ac- 

 tual rupture of these organs 

 themselves. It is upon this cir- 

 cumstance that the strength of 

 woody fibre depends, which is 

 very different in different plants. 

 It has been experimentally ascer- 

 tained, that the strength of silk, 

 New- Zealand flax (Phormium 

 tenax), hemp, and flax, are re- 

 spectively as the numbers 34 : 231, 



As the cells and tubes are of different lengths, their 

 extremities overlap each other, and thus as it were 

 dovetail the mass together. Wherever a transverse 

 fracture is most readily produced, as in the suture by 

 which a seed-vessel opens, or at the scar which is 

 c 4 



1H. 



