4 M. Mohl on the Circulation of the Sap 



considerable space in proportion to the cell ; so that, for instance, 

 when a whole series of cell-nuclei overlying one another length- 

 wise is formed in hairs, the intermediate space between the 

 individual nuclei is very narrow; and when subsequently the 

 horizontal walls have formed, each nucleus almost joins the upper 

 and lower horizontal walls of its cell. 



This relation generally disappears very soon, the cell either 

 expanding alone, or when the nucleus, which frequently happens, 

 grows even after the formation of the permanent cell-membrane, 

 the cell increases far more considerably in its relative size. 



The space between nucleus and cell-wall is at the commence- 

 ment, almost in all cases, entirely filled with the granular mucous 

 protoplasma. On treating such a cell with dilute tincture of 

 iodine, the protoplasma shrinks together, coagulating with the 

 assumption of a yellow colour ; and when the cell is already 

 somewhat advanced in its development, it does not solidify uni- 

 formly to form a dense globular mass, but in such a manner that 

 some smaller and larger roundish cavities are formed in its inte- 

 rior, which mostly run into one another at some points. The ap- 

 pearance of the cell is essentially modified by this. In its centre 

 is situated the nucleus surrounded by a thick layer of protoplasma, 

 its walls are in a similar manner coated with a layer of this sub- 

 stance, and between the two layers are some thicker or thinner 

 diagonal walls or columnar connecting pieces which maintain the 

 nucleus in its position and which traverse the cell diagonally. 



Analogous changes in the distribution of the protoplasma to 

 those which may be produced artificially by tincture of iodine in 

 the young cells, occur naturally in those cells whose develop- 

 ment is more advanced. Irregularly scattered cavities form in 

 the protoplasma, which become filled with aqueous sap. At first 

 these cavities are generally small and separated from one another 

 by thick layers of protoplasma ; but in other cases, likewise at an 

 early period, some larger cavities occur, while the remaining space 

 of the cell is still uniformly filled with granular protoplasma. 

 The older the cell and the more it expands, the more numerous 

 and large do these cavities become ; at first they are separated 

 from one another, and it has then frequently the deceptive ap- 

 pearance as if thin-walled cells filled with an aqueous fluid were 

 contained in the granular protoplasma. Two circumstances how- 

 ever prove the assumption that these bright spaces are sur- 

 rounded by membranes to be erroneous, however deceptively they 

 may frequently possess the appearance of cells. In the first 

 place, the protoplasma, when it flows out of an injured cell, ap- 

 pears as a viscous fluid which does not mix with the aqueous 

 sap of the cell, and whose cell-like spaces filled with the cell-sap 

 may be made to unite by moving backwards and forwards the 



