CONTINUITY OF PROTOPLASM. 27 



finally, the whole, or a portion of the originally cellulose 

 wall, may become converted into mucilage. This change is 

 very general in the algae^ also in some fungi, and portions 

 of the tissues of flowering plants. It is due to this mucilage 

 that the majority of algae, especially the youngest portions, 

 adhere so firmly to paper when dried. Mucilaginous tissues 

 become hard and horny when dried, but swell up very 

 considerably when soaked in water. 



The primitive thin cell-wall, although not perforated, 

 readily admits the passage of water through its substance to 

 the protoplasm in its interior, and one reason why the 

 internal thickening matter is locally deposited is obviously 

 for the purpose of securing the interchange of liquids and 

 gases necessary for the vital activity of the cell through the 

 unthickened portions of the wall. Another object of pits is 

 to allow of the contact of the protoplasm contained in 

 adjoining cells. The early conception of a cell was that of a 

 portion of protoplasm completely surrounded by a cell-wall, 

 and entirely isolated from the protoplasm of adjoining cells. 

 This is now proved to be incorrect ; the pits, or unthickened 

 portions of the walls of adjoining cells, are placed exactly 

 opposite to each other, and at these points the protoplasm 

 of the adjoining cells is brought into contact, this connection 

 between the protoplasm of adjoining cells is known as the 

 continuity of protoplasm. In the case of a single row of 

 superposed cells, as in many of the simpler red sea-weeds, 

 the protoplasm of adjoining cells is usually in contact at 

 one point only, situated in the centre of the transverse wall 

 separating the two cells ; in other examples where the cells 

 form a solid tissue, continuity of protoplasm between 

 adjoining cells usually takes place at several points. The 

 protoplasm of a more or less spherical cell, with thickened. 



