28 THE TISSUES OF PLANTS 
by the division of this cell the plant originates, but none- 
the-less the whole plant is to be considered as a unit and 
not as an association of distinct cells. 
38. In such higher plants we can distinguish several 
types of differentiated cells and can with correctness 
speak of tissues. A tissue may be defined as an associa- 
tion of similar cells for a common function. In the less 
differentiated plants the same tissue will have many 
different functions; in the more highly specialized forms 
there will be more kinds of tissues each with fewer func- 
tions. In the study of tissues we must distinguish 
between the so-called “false” and ‘‘true”’ tissues. The 
former are those that are formed by the subsequent close 
association of cells that originated independently of one 
another. Thus many separate motile cells (zoospores) 
may join themselves to one another in such a way as to 
form a definite structure (e.g. Hydrodictyon) or a sort of 
tissue may be formed by the growing together of numer- 
ous originally separate filaments of cells. On the other 
hand a true tissue is formed by successive divisions from 
one or a few cells, so that every cell may be said to have 
been formed in place. In the falsé tissues the walls 
between adjacent filaments or cells of different origin are 
double, without a true middle lamella while in true 
tissues the walls are single and the middle lamella is 
present (at least at first). It is sometimes impossible to 
make a very sharp distinction between these two kinds 
of tissues as one method of origin may be combined with 
the other. False tissues are found almost exclusively 
in the higher fungi and some of the algae while the tissues 
of the higher plants are true tissues. 
In the following discussion only the more highly 
differentiated types of tissues, such as occur in the higher 
plants, will be described in their main features while the 
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