88 



LECTURE VI. 



cells concerned have to fulfil in the life of the plant, both the chemical and the 

 physical properties of the cell-wall become changed. In the great variety here 

 met with, there may be distinguished three chief cases of metamorphoses of the 

 substance of the cell-wall as of especially frequent occurrence : these are lignifica- 

 tion, suberisation, and the conversion into mucilage. Lignification, which we find in 

 the typical form in the empty cells of ordinary wood, but also in many other cases, 

 is usually associated with a considerable thickening of the otherwise very thin cell- 

 walls. It is due to the formation of a peculiar chemical compound, which is 

 soluble in potash solution and also in a mixture of potassium chlorate and nitric 

 acid, and which causes the lignified cell-wall to become highly coloured when 

 treated with iodine solution and various other substances (e.g. with aniline 

 sulphate a bright yellow). It is to this woody substance, the so-called xylogen, 

 apparently, that the peculiar properties of the lignified cell-walls are to be as- 

 cribed. It is distinguished above all by its great hardness and elasticity, so 

 that it absorbs relatively but little water, and consequently increases but little in 



volume when thoroughly wetted, and on 

 drying up loses but little in volume — 

 properties upon which the endless varieties 

 of uses of wood in the arts especially 

 depend, and which in like manner come 

 into consideration for the life of the plant 

 itself Among the most prominent peculiar- 

 ities of lignified cell-walls (to which, more- 

 over, I shall return in detail at a later 

 period) is its capacity of allowing im- 

 bibed water to move with facility between 

 the molecules of the substance; upon which, 

 as we shall see, depends the physiological 

 significance of wood as an organ for con- 

 ducting water in transpiring land plants. 

 If the xylogen is extracted by means of 

 proper solvents from the cell-wall, a skeleton of the wall of similar form remains 

 behind, which then exhibits the ordinary reactions of cellulose. 



In a certain sense the suberisation of the cell-wall forms the contrast to its 

 lignification; it consists in that in the basis of cellulose another substance, 

 suberin or cork substance, is deposited. Such suberised cell-walls, of which 

 common bottle-cork, the skin of potatoes, the outer cortex or periderm of most 

 young branches of trees, &c. consists, may be somewhat thick ; they are generally, 

 however, relatively thin, upon which depends in part the compressibility of common 

 cork. The suberin, together with which are often found considerable deposits of 

 silica and other mineral substances in addition, confers upon the cell-wall the property 

 of being able to absorb water only in very small quantities; and of hindering with 

 great energy the passage of watery vapour and other gases through the cell-wall. In 

 a word, the suberised cells have essentially the properties to which common bottle- 

 cork owes its varied uses, because it allows the vapours of fluids to pass through it 

 so exceedingly slowly. On this account also every part of a plant the fluids of which 



Fig. 86, — Transverse section of the endosperm of Ccraiotu'a 

 sitiqua. a granular cell-contents; b solid walls surrounding 

 them; c the so-called intercellular substance — i.e. the middle 

 swollen portion of the wall between each two cell cavities 

 strongly magnified). 



