ELEMENTARY MORPHOLOGY AND PHYSIOLOGY. 9 
volume outstrips growth in mass. Cells are also known which 
possess no cell-wall, and this in all cases ae 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, 2.¢., the cell-wall is ¢y {i; 
kept on the stretch by pressure ~ 
from within. This turgidity, which 
causes the firmness of freshly-cut _ , 
flowers, and the want of which PR' 
makes them flaccid when faded, is Ly 
Fig. 1.—Sectional View of a Unicellular 
a phenomenon largely independent Plant, much magnified. [Original.] 
of life in the plant. It isa well- «1. Cw. cell-wall; Pr. protoplasm; 
Pee eal tack thabeik bwo NV. nucleus; n. nucleolus ; ch. chlo- 
poy rophyll bodies (granules), the black 
different liquids are separated by dots in which represent starch; V. 
E central vacuole, full of cell-sap; the 
a membrane which both can arrows on right indicate direction of 
moisten, diffusion currents will streaming movements in two proto- 
? 
z plasmic threads. 2, 3. Stages in divi- 
pass through the membrane in _ sion; in 2, the nucleus and protoplasm 
both directions, but not to an hasbeen formed; in's, the two halves 
equal extent. The gain in volume — 2e beginning to separate. 
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 
