558 CHEMISTRY IN AGRICULTURE AND PHYSIOLOGY. 
loss of that form the cell loses its peculiar vital characteristic 
reproductive powers. 
The basis of the substance of all vegetables, when examined 
by the microscope, is found to consist of cells; even in the 
most highly developed plants all the organs are in the 
youngest condition composed of cells alone, and the vessels 
only appear during the subsequent development. If a row 
of cells arranged in a line become combined during the 
course of their development into a tube, with an uninter¬ 
rupted cavity, through the absorption of their cross-walks, a 
compound elementary organ is produced —the vessel. The 
basis of the membrane of vegetable cells consists of cellulose, 
a colourless substance, which is insoluble in cold and boiling- 
water, alcohol, ether, and dilute acids. It is converted into 
dextrin by dilute sulphuric acid at boiling heat. When 
imbued with iodine, it becomes coloured indigo-blue. The 
credit is due to Payen of having demonstrated that the sub¬ 
stance of all cells, from the highest plants down to the 
fungi, when purified from foreign deposits, exhibits the same 
composition, and assumes the blue colours of cellulose on 
treatment with iodine and sulphuric acid. 
Cellulose probably does not occur in a pure condition 
in any cell-membrane, since a series of both organic and 
inorganic compounds are deposited within it; in which fact 
is to be sought the explanation of the manifold physical and 
chemical differences which are exhibited by the membranes 
of the same cell at different periods of their age, as well as 
by the cells of different plants. 
In all plants a skeleton (the ash) corresponding to the form 
of the membrane, and composed of the alkalies, earths, and 
metallic oxides which had been deposited in it, remains 
behind after the cells have been burnt. 
Since the corners and edges of cells are generally rounded 
off so that their flat faces meet at sharp angles in compa¬ 
ratively few cases, it follows necessarily from this condition 
that the cells are not coherent together by their whole sur¬ 
faces, but leave empty spaces between them, which run along 
the edge of the cells in the form of triangular canals, opening 
into each other at the corners of the cells, and forming a net¬ 
work of tubes branching throughout the whole plant, to 
which the name of intercellular passages has been applied. 
In living plants, they are, with few exceptions, filled with 
air. 
If a tissue composed of young cells be left some time in 
alcohol, a very thin membrane becomes detached from the 
inside walls of the cells in the form of a closed vesicle which 
