INFLUENCE OF THE MEDIUM 135 



be advantageous. At any rate, we may be sure that the 

 greater surface of the cylinder puts the plant into better con- 

 dition for exchange of material with its surrounding medium. 

 On the other hand, the more concentrated solution not only 

 withholds water from the cells, but presents a demand upon 

 them for water. The cell meets this in part by offering as 

 small a surface as possible to the solution. In this case, 

 although the requisite ions may be present, and even in the 

 right number, the scarcity of water in the protoplasm may so 

 decrease the lability that rapid growth is impossible. We 

 shall see that there is also a corresponding falling off in the 

 reproductive activity in strong solutions. Perhaps this 

 response is attributable to the increased general activity in 

 weak solutions. It has no relation to the form of the cell, 

 since zoospores are produced from both spherical and cylin- 

 drical cells, as well as from those of intermediate shape. 



It is to be emphasized that in the stronger solutions cell 

 division and growth are not only retarded, but the direction 

 of the dividing planes is curiously changed. Whereas in the 

 weak solutions the cylindrical cells divide only by walls in 

 one direction, the spherical cells of cultures in the more con- 

 centrated solutions divide in all directions. Whether this is 

 due to the change in form of the cell, or directly to the water 

 content of the protoplasm, cannot yet be decided. 



What may be the mechanics of the rounding up of cylin- 

 drical cells when placed in a concentrated solution is one of 

 the most important problems suggested by this research. 

 The fact that the dead cellulose membrane is almost entirely 

 reshaped during this process, without being dissolved, ren- 

 ders it probable that the change in form is directly caused by 

 some change in turgidity within the cell. In a rounding cell 

 the membrane moves and changes its form, and, since it is 

 entirely inert, the source of this motion must be either in the 

 activity of the protoplasmic body itself, or it must be in the 



