LIFE AND PROTOPLASM 



120 



forms ono of the most important corner stones in th.. roun.JMli.,,, of 

 biology. 



Wiiile plant and animal colls possess some rather striking cJifT.T- 

 ences m organization, they ar(> fnndamentally similar. Prac-tic-iUy 

 every cell that is microscopically visible possesses several difTerent 

 kmds of structures located within its borders. Some of these struc- 

 tures are alive, some lifeless. In the first group 

 may be placed the -plastiih of plant cells, the 

 mitochondria or chrondrio somes, some of which 

 probably give rise to plastids, fibers of various 

 kinds, the Golgi bodies and the centrosomes, the 

 latter of importance in animal cell division. In 

 the second group may be placed such inclusions 

 as yolk, or other food substances, fatty droplets, 

 granules of pigment or of secretions (as in gland 

 cells), and crystals of various kinds, such as 

 calcium oxalate in plant cells. To this list may 

 also be added vacuoles, which in plant cells often 

 occupy the major space within the cell membrane. 

 All of these structures are confined to the cyto- 

 plastn or part of the protoplasm outside the 

 nucleus. In Elodea, the cells present a green t^. 

 appearance, due to the presence of many tmy pi.yii cell of a K-af; 

 ovoid bodies, the chloroplasts, which are plastids <"• ihl()ro[)last ; n, nu- 

 containing chlorophyll. Careful obser^•ation of 

 a single cell shows that the chloroplasts move 

 slowly down one side of the cell, across one end, and uj) the other side, 

 keeping rather close to the outer edge of the cell during the process. 

 This is due to the movement of the cytoplasm. In the cells of the 

 hairlike stamen of Tradescantia, the movement of the cytopla.'^m is 

 also evident. Here it can be seen actively streaming in currents 

 within the cell, carrying along within it tiny crystals of inorganic 

 origin, as well as colorless plastids and granules. The latter term is 

 usually applied to inert materials, such as granules of stored food in the 

 form of starch grains (in plants), fat or yolk granules, or pigment 

 granules which frequently occur scattered througiiout |)r()toi)lasm. 

 Between the strands of (ytoplasm are spaces or vaeuolrs filled 

 with a watery fluid, called cell sap. In young jilant cells, the 

 vacuoles are small and the cytoplasm occupies the greater part 

 of the cell, but in mature plant cells the cytoplasm is found clo.se 



{'k'us ; r 

 u\ cell wal 



\ a<n(il(' 



