a 
ELEMENTARY MORPHOLOGY AND PHYSIOLOGY. 9 
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 as 
turgid state, 2.e., the cell-wall is Cu 
kept on the stretch by pressure 
from within. This turgidity, which n Sy 
causes the firmness of freshly-cut Pai ax ees 
flowers, and the want of which (4 ; 
makes them flaccid when faded, is a 
; = Fic. 1.—Sectional View of a Unicellular 
a phenomenon largely independent Plant, much magnified. [Original.] 
of lifein the plant. Itisa well- 1. C.w. cell-wall; Pr. protoplasm; 
H hale facoethaeatet N. nucleus; n. nucleolus; ch. chlo- 
nown physical fac at 1f two — rophyll bodies (granules), the black 
c FS Ale 7 ¢ dots in which represent starch; V. 
different liquids ane separated by central vacuole, full of cell-sap; the 
a membrane which both can arrows on right indicate direction of 
: = : aC streaming movements in two proto- 
moisten, diffusion currents will plasmic threads. 2, 3. Stages in divi- 
pass through the membrane in sion; in 2, the nucleus and protoplasm 
: 3 have divided, and a cellulose partition 
both directions, but not to an has been formed; in 3, the two halves 
equal extent. The gain in volume 2” 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, 7.¢., 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 
