TWO PATTERNS OF LIFE 7 



located the chromatin material which acts as the carrier of 

 inherited traits from one generation to the next. The cytoplasm 

 constitutes the bulk of the cell, and is that portion of the proto- 

 plasm surrounding the nucleus; it is generally more liquid in 

 consistency and in some plant cells can be seen carrying on a 

 slow streaming movement. It is in the cytoplasm that the living 

 activities of the organism take place, such as manufacture or 

 digestion of food, respiration and excretion. The cytoplasm 

 contains numerous inclusions some of which are living and some 

 not. The living inclusions, known as plastids, are chiefly special- 

 ized bits of cytoplasm which are self-perpetuating. Plastids are 

 characteristic of plant cells and have important functions. The 

 prevailing green color of vegetation is due to the presence of 

 plastids known as chloroplasts, which contain the green pigment, 

 chlorophyll. Other plastids of an orange, yellow, or red hue 

 (chromoplasts) are commonly found in fruits and flowers. Color- 

 less plastids (leucoplasts) act as depositors of starch for storage 

 purposes. Non-living inclusions are chiefly food particles, waste 

 products and water vacuoles. The latter are reservoirs of water 

 which look like small globules and frequently contain materials 

 in solution. Plant cells as a rule have much larger vacuoles than 

 animal cells have; sometimes the vacuoles occupy so much of 

 the cell that the cytoplasm merely lines the cell wall. In addition 

 to the cytoplasm and nucleus, there are the various membranes 

 which separate the different kinds of protoplasm. A nuclear 

 membrane separates the nucleus from the cytoplasm, and a 

 plasma membrane separates the living protoplasm from the 

 non-living cell wall. 



The Multicellular Organism 



In the early history of life the cell was the organism. But just 

 as naked protoplasm proved less satisfactory in the relentless 

 struggle for existence than protoplasm organized into cells, the 

 single-celled organism had to yield to the potentialities of 

 organisms made up of many cells united with one another. The 

 multicellular condition off'ers many advantages, among them 

 the possibility of attaining greater size, of adaptation to a wider 

 variety of habitats, of greater specialization of functions. Division 



