CELL CONTENTS. 41 
but in this connection we are justified in assuming that all these movements 
have to do with the maintenance and multiplication of the protoplasts. For 
instance, amongst the objects of the various movements are the search for food, the 
elimination of useless material, the production of offspring, the discovery of the 
rays of sunlight necessary to the existence of chlorophyll-bodies and of suitable 
spots to colonize. This conception has been brought out frequently in the course 
of the foregoing description, and will again engage our attention in succeeding 
pages. 
3. SECRETIONS AND CONSTRUCTIVE ACTIVITY 
OF PROTOPLASTS. 
Cell-sap.—Cell-nucleus.—Chlorophyll-bodies.—Starch.—Crystals.—Construction of the Cell-wall and 
Establishment of Communication between Neighbouring Cell-cavities. 
CELL-SAP.—CELL-NUCLEUS.—CHLOROPHYLL-BODIES.—STARCH.—CRYSTALS. 
In addition to the powers which the living protoplast possesses of shifting 
its parts, of expanding and contraeting, of dividing and of fusing like with like, 
it has also the properties of adapting different parts of its body to partieular 
functions, of building up various chemical compounds, and of separating them out 
when necessary. As the protoplast stretches and expands, spaces and depressions 
arise within it, and these form ultimately, when the protoplast is limited 
to a peripheral layer lining the walls of the cavity, a single central vacuole. 
In the spaces there is secreted, in the first instance, the cell-sap, a watery fluid 
containing a variety of substances either suspended or in solution, of which the 
chief are sugar, acids, and colouring matters. Moreover, in the interior of the 
protoplasm itself, structures with quite different forms occur, and are easily recog- 
nizable by their contours; these are the cell-nucleus, chlorophyll-bodies, and starch- 
grains. 
The principal feature of the cell-nucleus is that, although the substance of 
which it is composed is only slightly different from the general protoplasm of 
the cell, yet it is always clearly marked off from the protoplasm. In the un- 
developed protoplast the nucleus is usually situated in the middle, but in mature 
protoplasts it is either pressed against one wall of the cell or suspended in a sort 
of pocket of protoplasmic filaments in the interior (fig. 5! and 5°). It may 
be pushed along by the streaming protoplasm and dragged into the middle of 
the cell, and in that case its shape is sometimes altered and it becomes for a time 
somewhat elongated and flattened. The nuclear substance, which, as has been 
already mentioned, differs but little from ordinary protoplasm, is colourless, and 
studded with microsomata, and is liable to internal displacements similar to those 
of the entire cell-body. When a protoplast divides, the nucleus plays a very 
