42 
MORPHOLOGY OF THE CELL. 
for as much as half-an-hour or an hour (Fig. 40). This interpretation of the skin 
is not at all opposed to the fact that it is denser than the inner and more watery- 
substance. That the cohesion in each protoplasm-mass decreases from without inwards 
is shown by the greater mobility of the inner portion, especially with plasmodia, and 
also by the formation of vacuoles, which clearly depends on the collection of part of 
the water present in the protoplasm round internal points in drops, presupposing 
that the cohesion is overcome at these points. The view that the hyaline homogeneous 
basis itself forms on each exposed surface of the protoplasm the skin destitute of 
granules, entirely agrees with the supposition that not only every vacuole in a pro- 
toplasm-mass, but also every thread of protoplasm which traverses the sap-cavity, and 
finally the sap-cavity itself, is also bounded by a skin, even if it be so thin that it cannot 
be seen when strongly magnified ^ 
If the protoplasm is not enclosed in a cell- wall, the vacuoles are usually small and 
not numerous. If, on the other hand, a cell-wall is formed, and if the cell grows 
rapidly, this is always accompanied by an 
increase in number and size of the vacuoles 
(Fig. T, p. 2). This not unfrequently leads 
to a frothy condition of the protoplasm where 
the vacuoles are separated only by thin lamellae 
of that substance (Fig. 41, J) \ but in other 
cases the inner protoplasm-mass of a cell 
breaks up into smaller portions, each of which 
encloses a large vacuole surrounded by a thin 
skin of protoplasm (Fig. 41, B, b). These 
are the ' Sap-njesicles ' which are so com- 
mon, and which sometimes enclose granules 
of chlorophyll and other substances, and thus 
resemble cells ; they are not uncommon in the 
flesh of berry-like fruits, and in tissues with 
mucilaginous juices. If the rapidly growing 
cell does not form new protoplasm as the size 
of the cell and the amount of sap increase, 
the quantity of protoplasm decreases ; and not 
unfrequently it forms a thin sac not im- 
mediately visible, lying between the cell-wail 
which it lines and the cell-sap which it in- 
vests, and becoming visible only by means of 
reagents that remove the water and separate 
the protoplasm-sac — the Primordial Utricle of 
Mohl2_from the cell wall by contraction (Fig. 41, C,p). The significance of this thin 
protoplasm- sac, and its production owing to the increase in number and size of the 
vacuoles in an originally continuous protoplasm-mass, will no longer be doubtful to 
the reader after all that has been said in Sects, i, 2, and 3, and by comparison of 
Fig. I with Fig 41. 
In young cells, where the protoplasm still forms a thick layer or a net-work per- 
meated by vacuoles, its substance— with the exception perhaps of the outermost layer 
lying on the cell-wall— appears to be always engaged in a 'streaming' movement, which 
is however usually very slow. In many mature and large cells, which do not serve 
for the storing up of assimilated materials, and where the protoplasm-mass is sufficiently 
nourished, and does not, as the cell increases in size, contract to a mere thin skin, 
Fig. 41. Forms of tlie protoplasm contained in cells. 
yl and B of maize ; A cells from the first leaf-sheath of 
a g-erminating plant ; B from its first internode ; Cfrom the 
tuber of the Jerusalem artichoke, after action of iodine 
r.nd dilute sulphuric acid ; h cell-wall ; k nucleus ; / pro- 
toplasm (primordial utricle). 
1 See Hanstein, Die Bewegungserscheinung des Zellkerns, u. s. w. in Sitzungsberichte der nieder- 
rheinischen Gesellschaft zu Bonn, Dec. 19, 1870, p. 224. 
^ [H. von Mohl, Bot. Zeitg. 1844, p. 273.] 
