CELL CONTEXTS. 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 CONSTEUCTIVE ACTIVITY 

 UF PROTOPLASTS. 



Cell-sap. — Cell-nucleus. — Chlorophyll-bodies. — Starch. — Crystals. — C'oustructiou of the Cell-wall :incl 

 Establishment of Communication between Neigh boui'ing 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 contracting, of dividing and of fusing like with like, 

 it has also the properties of adapting different parts of its bod}- to particular 

 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 diflerent 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 oft' from the protoplasm. In the vni- 

 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, difters but little from oi'dinary 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 



