ELEMENTARY STRUCTURE. 9 



much more extended longitudinally, some having been measured 

 as much as i of an inch long, and there is no doubt but that 

 those of the inner bark of the flax and hemp plants are even 

 much longer. The prosenchymatous cells of the wood and 

 inner bark of trees generally vary however, from about the ^^ to 

 the pj of an inch in length. 



Those cells again which have an unrestrained development 

 are frequently also far more extended in length. Thus, the cells 

 of which cotton is formed are sometimes as much as one or two 

 inches long, while in the lower Alga, as Chara, cells occur 

 several inches in length. 



3. The Cell-membraxe or Cell- wall. — a. Its Chemical 

 Properties. — The membrane of which the cell is composed consists 

 of the substance called cellulose, and as all plants and all parts of 

 a plant are formed essentially of cells variously modified and com- 

 bined, this substance must be considered as the fundamental mate- 

 rial of the plant. When pure it is a ternary compound of carbon^ 

 hydrogen, and oxygen ; of which the latter two exist in the same 

 proportions as in water. Hence it may be considered as con- 

 sisting of carbon and the elements of water. The formula of 

 cellulose, according toPayen, is C12 Hio Oio; it is thus closely 

 alhed in composition to starch, if not actually isomeric with it. 

 According to Mulder, however, the formula of cellulose, as shown 

 by the experiments of Fromberg, is not C12 ^\o Oio, but C24 

 H21 ©21 ; agreeing, not with starch, but with soluble inuline, 

 which has a similar composition. The formula of Payen is that 

 however which is generally adopted. 



Cellulose is insoluble in both cold and boiling water ; also in 

 alcohol, ether, and dilute acids, and almost insoluble in weak 

 alkahne solutions. By the action of concentrated sulphiiric 

 acid and caustic potash, it is changed into dextrine. When 

 iodine and sulphuric acid are applied to it, it assumes an indigo 

 blue color, which is rendered more evident if the sulphuric 

 acid be previously diluted with water (the best proportions being 

 one part of the latter to three of the former). A similar blue 

 colour is also produced, as first shown by Schultz, when cellulose 

 is moistened with a solution of chloride of zinc, iodine, and 

 iodide of potassium.* Mohl has also shown that cellulose will as- 

 sume a blue colour if it be thoroughly imbued with iodine, and 

 afterwards moistened with water. The blue colour will not be 

 produced however, under such circumstances, when we operate 

 upon the cell-membrane of very young cells, hence it is probable 

 that the young cell-membrane may be composed of a substance 



* This solution may be thus prepared. — Dissolve zinc in hydro- 

 chloric acid ; then evaporate the solution in contact with metallic 

 zinc to the thickness of a syrup ; the syrup is then to be 

 saturated with iodide of potassium ; after which iodine is 

 added, and the solution, when necessary, is diluted with water. 



