STRUCTURE.] ITS STRENGTH AND SIZE. 63 



organic fibres. He found that, in suspending weights to 

 threads of the same diameter, 



Silk supported a weight equal to ... 34 



New Zealand flax 



Hemp 



Flax Ill- 

 Pita flax (Agave Americana) .... 7 



That even the most delicate woody tissue consists of tubes, 

 may be readily seen by examining it with a high magnifying 

 power, and also by the occasional detection of particles of 

 greenish matter in its inside. A very different opinion was 

 nevertheless once held by some physiologists, who thought 

 that woody tissue is capable of endless divisibility. " When/' 

 says Duhamel, " I have examined under the microscope one 

 of the principal fibres of a pear tree, it seemed to me to con- 

 sist of a bundle of yet finer fibres ; and when I have detached 

 one of those fibres, and submitted it to a more powerful mag- 

 nifying power than the first, it has still appeared to be formed 

 of a great number of yet more delicate fibres." (Physique 

 des Arbres, i. 57.) To this opinion Du Petit Thouars assents, 

 conceiving the tenuity of a fibre to be infinite, as well as 

 its extensibility. (Essais sur la Vegetation, p. 150.) These 

 views arose from the use of bad microscopes ; under low 

 powers of which such appearances as Duhamel describes are 

 visible ; but with modern glasses, and after maceration, each 

 particular tube can be separated with the greatest facility. 

 Their diameter is often very much less than that of the finest 

 human hair ; the tubes of hemp, for example, when com- 

 pletely separated, are nearly six times smaller. It must, 

 however, be observed, that the fibres of this plant, as used in 

 linen-making, are by no means in a state of final separation, 

 each of the finest that meets the naked eye being in reality a 

 bundle of tubes. While some do not exceed 3 1 of an inch 

 in diameter, others have a diameter as considerable as that of 

 ordinary cellular tissue itself; in Conifers the tubes are often 

 -5^0- or -3-^-0, and in the Lime they average about -pf^. Link 

 states (Elementa, p. 85.) that they are very large in trees of 

 hot countries. The sides of woody tissue become thickened, 



