24 THE PROTOPLASM, 
glucose. When cellulose is steeped in dilute sulphuric acid, and 
then treated with a solution of iodine, or if it is acted upon by 
Schulze’s solution of iodine in zine chloride, it acquires a more 
or less blue colour. The cell-wall contains in addition to the 
molecules of cellulose a small quantity of mineral ash. 
li rarely happens that cellulose can be found pure, as, in addi- 
tion to the mineral ash above mentioned, it generally is rendered 
more or less impure by the protoplasm which remains after the 
death of the cell. That which is furnished by the cells of hairs, 
such as Cotton, is generally the most free from extraneous 
matters. The cell-wall is frequently hardened by the conversion 
of its cellulose into a substance called lignin. This lignification 
takes place where hardness or strength is required, as in the 
tissue forming the shell of nuts, or in the elongated cells of the 
wood of trees. The outer walls of cells also, which he on the 
surface of plants, and are consequently exposed to more active 
chemical influences, usually become cuticularised (see page 59), 
as in the epidermis of leaves and in the cork cells of the bark ; 
the cell-wall in such cases becomes thickened and impervious 
to moisture, and it is owing to this circumstance that delicate 
plants are enabled to withstand the scorching and withering 
heat of the hot sun; itis also this cuticularisation of the cork 
cells of the bark which protects the internal living parts of trees 
from the damaging influence of frost in winter, 
Besides the above-mentioned changes which take place in 
the cell-wall, others occur which are the result of degradation. 
The mucilage of plants, as that of the Mallow, or the slimy sub- 
stance given off by Seaweeds, or the gelatinous matrix of such 
organisms as Nostoc and Gleocapsa, are examples of this ; gums 
and resins are also the products of the degradation of the cell- 
walls of special cells of the wood of the trees in which they occur. 
2. THe Prortoptaso is the only part of the cell, and there- 
fore of the whole plant, which is possessed of life ; and the differ- 
ences in the form, size, and nature of cells is due to the vital 
energy which it is capable of exerting. If this energy is exerted 
equally in all directions, and there are no other counterbalancing 
forces, such as pressure from neighbouring cells, the form which 
the cell will assume will be one approaching to a sphere (jig. 62). 
If, on the other hand, this energy is exerted in one direction only, 
the cell will assume an elongated form (jig. 70). If again in two 
directions, flattened or tabular cells will be the result (fig. 68) 
(see Forms of Cells). This internal energy, which is peculiar to 
living protoplasm, is frequently spoken of as vital force. 
The appearance of protoplasm is as varied as is the form of 
the cells which it produces. It may be granular and somewhat 
opaque, or perfectly transparent ; it may be almost fluid, or of 
the consistency of dough ; or again it may be stiff or even brittle ; 
generally, however, it is of a light grey colour and more or less 
granular ; but it is never a true fluid. In those cases where 
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