20. INTRODUCTORY. 



65 



continuity of the protoplasm] that is, that the protoplasm of one 

 cell is connected with that of the contiguous cells by means of very 

 delicate protoplasmic fibrils which traverse the pits or pores of the 

 intervening cell-walls (Fig. 35). This connection appears, how- 

 ever, to exist from the first development of the cells, and thus 

 differs from the case of the syncyte where the absorption of the 

 intervening cell-walls is a secondary process. 



The term tissue is generally applied to any continuous aggregate 

 of cells ; but it is essential to define the term more accurately. A 

 true tissue is an aggregate of cells which (1) have a common origin, 

 whether formed simultaneously (e.g. development of endosperm of 

 Phanerogams), or successively, as in the case of a tissue developed 

 from a growing- 

 point ; which (2) are 

 coherent from the 

 first and are 

 governed by a com- 

 mon law of growth ; 

 and which (3) are 

 physiologically in- 

 terdependent and 

 cannot, in fact, exist 

 otherwise than as 

 part of the tissue. 



The tissue of 

 which the body of a 

 plant consists may 

 be either homo- 

 geneous or hetero- 

 geneous ; that is, 

 the cells may be all 

 alike, constituting 

 therefore but one kind of tissue ; or they may not be all alike, 

 the different kinds of cells being more or less grouped together so 

 as to form different kinds of tissue. A body which consists of 

 different kinds of tissues is said to be histologically differentiated. 

 The structural differences between the various forms of tissue in a 

 histologically differentiated body are essentially connected with the 

 special adaptation of each form of tissue to the performance of some 

 particular function in the economy. 



It is a remarkable fact that, whilst the cells of the various 



M.B. F 



FIG. 35 (highly magnified, after Gardiner). Continuity 

 of the protoplasm of contiguous cells of the endosperm of 

 a Palm-seed (Bentinckia) : a contracted protoplasm of a 

 cell; b a group of delicate protoplasmic fibrils passing 

 through a pit in the cell- wall. 



