A POND SCUM 41 



of slimy cytoplasm (also difficult to see) surrounds the nucleus, and 

 strands of the same material connect the layer about the nucleus with 

 the layer next the wall. Thus all parts of the slimy cytoplasm are 

 connected with one another, and the nucleus and chloroplast are im- 

 bedded in it. 



The rest of the space inside the wall that is not occupied by the 

 slimy cytoplasm, chloroplast, and nucleus is the central vacuole. From 

 the description that has been given it is plain that this vacuole is of 

 very irregular shape. It contains a clear liquid (cell sap), composed 

 of water containing a weak solution of salts, sugars, acids, and other 

 substances. 



62. The Food of Spirogyra. The word food may be ap- 

 plied to any substance that is used by a living cell in any 

 stage of the building up of living matter. This building-up 

 process is one of many steps. Therefore there is a long series 

 of substances that may be used by plants, beginning with very 

 simple things (water and carbon dioxid), and ending with 

 certain very complex ones (proteins), which represent the last 

 steps previous to the formation of living matter itself. In 

 taking in food substances from outside, and in using these 

 substances within its own body, Spirogyra behaves very much 

 as does Protococcus and as do the higher green plants (such 

 as the cucumber) with which we are more familiar. In the 

 higher green plants, the work of absorbing and of manufac- 

 turing food is divided between many different kinds of cells ; 

 but in Spirogyra, as in Protococcus, each cell does practi- 

 cally all this work for itself. Spirogyra, like all other plants, 

 can take in materials from the outside only in a dissolved 

 condition ; everything that is to be absorbed, therefore, must 

 be dissolved in water before it can pass through the cell wall. 



63. Manufacture of Carbohydrates. It is true of all 

 green plants that they take in from the outside world only a 

 few very simple substances and that they build up these sub- 

 within the cell wall, and cytoplasm to all of the protoplasm excepting the nucleus. 

 Using the terms in this way, the chloroplasts, vacuoles, and some other parts of the 

 cell are parts of the cytoplasm, as is also the slimy, viscous material here called the 

 slimy cytoplasm. 



