2 PLANT LIFE. 



3. Protoplasm. — The protoplasm is the essential part of 

 the cell. It constructs the cell-wall. Rarely, if ever, is it 

 uniform throughout, but is differentiated into distinct mem- 

 bers, each having special work to do. In the most com- 

 pletely differentiated active cells the 

 greater part of the protoplasm con- 

 sists of a finely granular or nearly 

 transparent, colorless portion, called 

 cytoplasm. Embedded in the cyto- 

 plasm are the nucleus, centrospheres 

 (figs, i and 2), and plaslids (figs. 

 3 to 8). 



Fig. i.-A cell (the megaspore) 4. Cytoplasm. This is not a 



from a lily ovule, filled with • i 1 A ■% . • . ,- 



granular protoplasm, m which single substance, but a mixture of 

 nlckus e , dd c^ several different substances, so ia- 



^r^rZTr^rZl timately mixed and so unstable that 



around the protoplasm repre- • , • , .1 , , 



sents the ceii-waii. with those it is not possible to analyze It. 



ed. the M a agnffi?d "io diam!- Moreover, the nature and amount 

 of the components are probably 

 variable. Most of the substances belong to the class of com- 

 pounds called proleids, so that cytoplasm responds to proteid 

 tests and is often spoken of as a mixture of proteids. In 

 addition there are frequently present other organic substances 

 (such as amides, carbohydrates, fats, and enzymes), and 

 always small quantities of mineral matters which appear as 

 ash when cytoplasm is completely burned. The minute 

 granules embedded in the cytoplasm are of various nature. 

 Most of them are solid substances. 



5. Vacuoles. — Scarcely distinguishable from these at first 

 are the minute cavities, called vacuoles, filled with dilute 

 watery solutions of many different substances, the cell-sap. 

 In all but the youngest cells more or fewer of these bubbles 

 of water may unite to form larger ones (fig. 7). These often 

 increase so as to occupy the greater part of the space within 



