
INT RODUCTION-10 PLANT HISTOLOGY 49 
the contents of one cell can pass into the adjoining one, as 
in Fig, 15, III. The cell-wall then 
remains as a plate. This form ot 
vessel is known as a steve-tube, and 
the perforated cell-wall is called a - 
sieve-plate (see Fig. 16). 
In one or other of the ways 
just described tissues are formed. 
Although each tissue has its de- 
finite and special work to do, it 
must never be forgotten that a 
plant is a living whole, each 
tissue contributing to the health 
and general well-being of the 
- organism. 
SUMMARY. 
1. All plants, and all parts of a 
plant, consist of cells. 
2. Cells are grouped together into 
tissues by a process of differentiation, 
each tissue having its special work. 
3. Tissues may be divided into — 
(a) Meristematic—-those which 
retain the power of growth. 


Fic. 16.—DIAGRAM SHOWING 
Stmve- Tuspes IN Woop 
VESSEL. 
I, II, Sieve-tubes with sieve- 
plates; s.p, sieve-plate ; III, a 
wood vessel with portion of 
wall removed to show interior. 
| (b) Permanent—those which have lost the power of growth. 
4. The changes by which Meristematic tissues are converted into 
Permanent are— 
(i.) Change in shape of the cell, the tissue formed being either 
parenchyma or prosenchyma. 
(ii.) Change in the thickness and chemical composition of the 
cell-wall, as in lignified wood cells and in cork cells. 
(iii.) Change in cell-unions, by which vessels are formed. 
~ 
