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 few^er func- 

 tions. In the study of tissues we must distinguish 

 between the so-called ''false" and 'Hrue" 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 false 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 



