60 ELEMENTARY MEMBRANE AND FIBRE. 



fibre, all the tissues of plants may be regarded as consisting, 

 when they are newly formed, is of variable thickness and trans- 

 parency In general, however, it is quite sufficiently trans- 

 parent, to allow the colour of fluids in contact with it to be dis- 

 tinguished on the other side ; and accordingly, though itself 

 colourless or nearly so, it often appears tinged, in consequence of 

 the cells or vessels which it forms, being filled with coloured 

 fluid. Thus the cells of leaves appear green, those of the parts 

 of flowers yellow, blue, red, &c. ; not because that colour exists 

 in the membrane of which they are composed (which, if they 

 could be emptied, would appear almost colourless), but on 

 account of the minute colouring particles diffused through their 

 contained fluids. One of the most remarkable properties of 

 vegetable membrane, is its power of allowing fluids to pass slowly 

 through it, even though no visible pores or apertures can be 

 detected in it. Occasionally the appearance of such apertures 

 exists, when membrane is highly magnified ; but this appear- 

 ance is sometimes produced by grains of semi-transparent matter 

 sticking to it ; and is sometimes due to that portion of the mem- 

 brane being thinner than the rest, through the deposition of new 

 matter upon certain points, subsequent to the first formation, of 

 which several examples will be presently given.' 



70. Elementary Fibre may be compared to hair of extreme 

 delicacy ; its diameter often not exceeding the 1-12,000 of an 

 inch. It is generally transparent and colourless, and is usually 

 disposed in a spiral direction. Its peculiar property is elasticity, 

 combined with a degree of firmness, which, for its diameter, is 

 very considerable. Accordingly we find its chief use to be, the 

 keeping open, like an interior spring, the delicate membranous 

 tubes through which air is to pass, and the preventing these from 

 being pressed together by the growth of neighbouring parts. 

 Not unfrequently, however, it seems less elastic than usual, and 

 is broken during the processes of growth, into several smaller 

 fragments ; which then exhibit a peculiar tendency to grow 

 together, in various irregular forms. In this way, several pecu - 

 liar kinds of tissue are produced, which will shortly be noticed. 



7J. The one most universally present, no kind of plant being 



