21.] 



INTRODUCTORY. 



91 



There is another kind of structure occurring in cellular plants 

 which has to be distinguished from both the cell and the coenocyte : 

 that is the syncyte. This structure is developed from already- 

 formed cells by an absorption, more or less complete, of the cell- 

 walls, which places the cavities of the adjacent cells in direct con- 

 tinuity. The commonest case of this occurs in the development of 

 vessels, where the transverse septa of a longitudinal row of cells 

 are absorbed so that a continuous tube is formed. 



But even in the fully-developed cellular plant-body it appears to 

 be very frequently the case that the energid in one cell is not 

 absolutely cut off from those of the adjacent cells, but that there 

 is continuity of the pro- 

 toplasm ; 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. 45). This con- 

 nection appears, how- 

 ever, to exist from the 

 first development of the 

 cells, and thus differs 

 from the case of the 

 syncyte where the ab- 

 sorption of the inter- 

 vening cell-walls is a 

 secondary process. 



The term tissue is generally applied to any continuous aggregate 

 of cells (or of coenocytes) ; but it is essential to define the term 

 more accurately. A true tissue is an aggregate of cells (or of 

 ccenocytes) which (L) 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 grow- 

 ing-point ; which (2) are coherent from the first and are governed 

 by a common law of growth ; and which (3) are physiologically- 

 interdependent and cannot, in fact, exist otherwise than as part 

 of the tissue. 



r^7_^-4 a 



Fio. 45 (highly magnified, after Gardiner). Con- 

 tinuity of the protoplasm of contiguous cells of the 

 endosperm of a Palm-seed (JBentinckia) : a contracted 

 protoplasm of a cell; b a group of delicate protoj lasmic 

 fibrils passing through a pit in the cell-wall. 



