64 WOODY FIBRE. 



of the most interesting modifications of it, is found in the Sphag- 

 num or Bog-Moss ; and in the coverings of some seeds. This 

 consists in the presence, within the membranous wall of the cell, 

 of a spiral fibre, coiling from one end to the other. In some of 

 the seed-coats in which these spiral cells exist, the membrane of 

 the cells is so delicate as to be easily dissolved away ; so that, if a 

 portion be put into water, the fibres spring out very beautifully 

 by their own elasticity. 



76. The next form of elementary tissue to be described, is that 

 called Woody Fibre. It has received the name of fibre, because 

 it always exists in an elongated form, and several 

 of the tubes of which it consists, adhere together conti- 

 nuously, so as to form cords. This is seen in the common 

 flax thread, for example. If the finest thread that could 

 be separated with the naked eye, were submitted to a 

 microscope, it would be seen to consist of several other 

 fibres adhering together ; none of these have any great 

 length ; but by the manner in which they adhere, side 

 by side, and end to end, a continuous cord is produced. 

 Each of these minute fibres, when more closely examined, 

 is seen to consist of a slender transparent tube, tapering 

 to a point at each end. It thus resembles a greatly 

 elongated cell. It differs from cellular tissue of similar 

 BUNDLE form, in the much greater strength of the membrane 

 K FiI DY forming the walls of the tubes >* though it is at the 

 same time thinner. There are many intermediate 

 forms, however, between one and the other. Woody fibre is 

 evidently destined to convey fluid in the direction of its length, 

 and is easily permeated by it. Minute openings have sometimes 

 been detected, in the points of the tubes, so as to connect one 

 cavity with another, and thus to render the passage of fluid 

 more easy. It is, however, especially destined to give firmness 

 and elasticity to the parts of the structure which require sup- 

 port ; and we almost constantly find vessels protected by it, 

 wherever they exist. 



77. In all plants with permanently- elevated stems, this tissue 

 is very abundant in the adult state. It forms a large proportion 



