30 BOTANY OF THE LIVING PLANT 



ends are partitioned off from the adjoining cells by transverse septa 

 marked by shallow pits, a condition which is usual for the septa, or 

 party-walls between cells. Protoplasmic connections probably pass 

 through these pits. Each protoplast (p. 1 8) is thus enclosed by an 

 elastic envelope of cell-wall, composed chiefly of cellulose and normally 

 saturated to a greater or lesser extent with imbibed water. A film 

 of cytoplasm lines the whole internal surface of the cell-wall, and thus 

 completely invests the large internal cavity, or vacuole, which is 

 filled with watery cell-sap. A highly refringent nucleus (as a rule 

 more sharply delimited than in the Figure), surrounded by a sheath of 

 granular cytoplasm, is suspended in a central position by threads of 

 cytoplasm, which traverse the vacuole. Their arrangement is irre- 

 gular, with frequent branchings ; but most of them converge towards 

 the nucleus, which is thus indicated as a functional centre of the 

 whole protoplast. Granules and inclusions of various sorts are seen 

 immersed in the cytoplasm. The largest of these are the plastids, 

 which in this cell are green and are termed chloroplasts, but many 

 other smaller bodies are present, including occasional crystals. The 

 nucleus, cytoplasm and plastids are the actual living components of 

 the cell, and collectively form the protoplast. 



In many living cells the protoplasm exhibits streaming movements. 

 It is probable that the movement is initiated in the cytoplasm, the 

 plastids and other cell-inclusions being carried along passively. The 

 plastids may be carried back and forth along the threads that suspend 

 the nucleus, as in the cucumber-hair cell, or along the cytoplasmic 

 lining of the cell. 



Properties of the Living Cell. 



Of the various properties of the typical living plant-cell, that of 

 effecting chemical transformation is one of the most prominent and 

 important. In connection with the nutrition of the plant there 

 proceed within its constituent cells very diverse chemical reactions, 

 complex substances being elaborated from simple raw materials 

 derived from the environment of the plant. Some of these activities 

 are restricted to particular cells in the Higher Plants : thus the 

 process of photosynthesis (see Chapter VIII.) occurs chiefly in the 

 cells of the leaf. Other types of chemical activity are exhibited by 

 all living cells. These chemical activities, involving also transforma- 

 tions of energy, are grouped together under the general term Meta- 

 bolism, and are considered in more detail in Chapter VIII. Growth 



