ELEMENTARY MORPHOLOGY AND PHYSIOLOGY. 



Fig. i. — Sectional View of a Unicellular 

 Plant, much magnified. [Original.] 

 i. C.u\ cell-wall ; Pr. protoplasm ; 

 N. nucleus; ?i. nucleolus; ch. chlo- 

 rophyll bodies (granules), the black 

 dots in which represent starch ; V. 

 central vacuole, full of cell-sap ; the 

 arrows on right indicate direction of 

 streaming movements in two proto- 

 plasmic threads. 2, 3. Stages in divi- 

 sion ; in 2, the nucleus and protoplasm 

 have divided, and a cellulose partition 

 has been formed ; in 3, the two halves 

 are beginning to separate. 



volume outstrips growth in mass. Cells are also known which 

 possess no cell- wall, and this in all cases appears to be formed 

 by the protoplasm, probably from 

 transformation of a surface layer. 

 Another important point is that 

 the active vegetable cell is in a 

 turgid state, i.e., the cell- wall is 

 kept on the stretch by pressure 

 from within. This turgidity, which 

 causes the firmness of freshly-cut 

 flowers, and the want of which 

 makes them flaccid when faded, is 

 a phenomenon largely independent 

 of life in the plant. It is a well- 

 known physical fact that if two 

 different liquids are separated by 

 a membrane which both can 

 moisten, diffusion currents will 

 pass through the membrane in 

 both directions, but not to an 

 equal extent. The gain in volume 

 will be on the part of the liquid, usually the denser one, which 

 can wet the membrane more readily. The phenomenon is 

 called osmosis, the passage inwards being endosmosis, that outwards 

 exosmosis. These processes can be conveniently studied in an 

 artificial cell, constructed in the following way : — A short piece of 

 fairly wide glass -tubing, filled with a solution of sugar, is closed at 

 both ends by vegetable parchment, and placed in water. Osmotic 

 currents are set up, but more water passes into the denser sugar 

 solution than vice versd, and the result is that the elastic parch- 

 ment ends bulge out and are placed on the stretch. Some sugar, 

 however, does diffuse out, and slightly sweetens the water outside. 

 The natural cell turgesces in a similar way, but, of course, is 

 bounded by an extensible membrane on all sides. If, at the end 

 of this experiment, the external fluid is made denser than the in- 

 ternal, a flattening of the membranes takes place. Such flattening 

 might also be caused by evaporation through the membranes. We 

 now see in what way the ideal plant can feed, i.e., by taking in 

 osmotically water that contains various salts and carbon dioxide 

 in solution. The chemical nature of the liquids largely affects the 

 rapidity of the osmotic currents, and the cell contains in its sap 

 osmotically-active substances {e.g., organic acids), which increase 

 endosmosis to a large extent. 



Having considered the reception of food, the next point to be 

 dealt with is the exact nature of that food. Information on this 



