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INTRODUCTION TO PLANT HISTOLOGY 47. 
vital activities of the plant, 
especially with nutrition. If, 
however, the cell gets longer 
than it is broad, the tissue is 
called prosenchyma. As these 
cells grow their ends be- 
come pointed and their walls 
thickened, then fibres are 
formed. These prosenchyma- 
tous fibres are chiefly found 
in those tissues whose func- 
tion it is to support the plant. 
The changes that take place 
in the thickness and chemical 
the groundwork of plants, and 1s le concerned in Jee 

Fic, 12,-CELLS oF PARENCHYMA, 
CONTAINING STARCH GRAINS. 
composition of the cell-wall are mainly of three kinds: 
(a) A substance, called lignin, may be deposited in the cell- 
wall; then the cells are gradually converted into wood. This 

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¢.W, lignified cell-wall; c¢.c, cell- 
cavity. 
takes place more or less in all 
flowering plants. A cell-wall 
which has become lignified 1s 
very permeable, so that it 
easily allows of the passage 
of water. A thin slice of 
a wooden match, examined 
under the microscope, shows 
cells with lignified walls. 
(b) The cell-wall may be- 
come suberised (Gk. suber, 
cork), then cork is formed. 
This tissue is not permeable, 
so that it is found in those parts of a plant which are exposed 
to the air, and which protect the inner tissues. 
Cork is, 
therefore, largely found in the stems of woody plants, 
in the trunks of trees, etc. 
shows the shape and appearance of cork cells. 
A thin slice of bottle-cork 
In the case 
of both wood and cork, it should be noticed that the cells 
