GROWTH OF CELLS. 147 



expansion of the membrane depends solely on the nutrition 

 of the lateral faces, a part of the latter having become 

 end-face ; but for this hypothesis we require a displace- 

 ment of particular parts of the membrane, which has 

 never yet been demonstrated in the cells of plants. Be- 

 sides, I will show by another analogous example that 

 such displacement really does not exist. 



According to Schleiden, the elongated cells owe their 

 shape to a current of sap, their ends being thus more nou- 

 rished. On the other side it is to be objected, that if a 

 cylindrical or prismatic cell is more nourished at its end 

 than at its lateral faces, the cell will become proportionately 

 broader not proportionately longer, since the ends deter- 

 mine the breadth of the cylinder or prism. But, according 

 to the possibility mentioned in the previous example, a por- 

 tion of the ends might continually become lateral faces ; so 

 that the ends would always remain the same size, while the 

 lateral faces would grow by the addition of new portions 

 of membrane, especially at both extremities (that is, near 

 the two end faces). That this is not the case, however, 

 is proved by such cylindrical or prismatic cells as have 

 fixed points, on their lateral faces, from the conditions of 

 which we may judge of the expansion of the membrane. 

 The points of attachment of the branch cells are fixed 

 points of this kind, in plants and organs of very simple 

 structure. In Confervacese and Callithamniaceae the 

 branches often originate at a very early period ; they are 

 attached to the upper ends of the cylindrical lateral faces. 

 Now, I observed in several cases that the superficial con- 

 tents of the lateral faces expanded ten to twenty times 

 more than the end faces, after the origin of the branch, 

 and that during this the branch always either remained 

 near the end, or was but slightly removed from it. The 

 lateral face is free,, and in contact with water ; the end is 

 in contact with a cell. According to Schleiden's theory, 

 the end face ought to be more nourished than the lateral, 

 since it effects the exchange of organic substances, while 

 the latter only serves for the absorption of inorganic 



