INTROD VCTION. 



out, but shows distinct parts, each having special work to do. 

 In the most complete and active cells the greater part of the 

 protoplasfti consists of a finely granu- 

 lar or nearly transparent, colorless 

 portion, in which the other "parts 

 seem embedded. 



Protoplasm is not a single sub- 

 stance, but a mixture of several dif- 

 ferent substances, so intimately 

 mixed and so easily destroyed that 

 it is not possible to analyze it. More- 

 over, the nature and amount of the 

 components are probably variable. 

 In all but the youngest cells there are 

 one or more bubbles of water in the 

 protoplasm. 



5. Nucleus. — The nucleus is one 

 of the most important parts of the 

 cell. It is generally spherical or ovoid, but in long cells it 

 may become elongated (fig. 2, 0). The nucleus may divide 

 into two, and this is commonly followed by the formation 

 of a partition-wall separating the cell into two parts, each con- 

 taining one of the daughter-nuclei. 



6. Plastids. — In most cells there are also other parts, 

 called plastids. In young cells these are small, rounded, 

 colorless bodies. As the cell grows older they increase in 

 size and number.. When mature and in cells which lie near 

 the surface of green plants, they are commonly roundish- or 

 biscuit-shaped, of spongy texture, and colored yelllowish- 

 green by a substance known as chlorophyll . These are con- 

 sequently known as chloroplasts or chlorophyll-bodies (fig. 

 2). In other cells, particularly those for the storage of food, 

 they may develop into smaller, denser, flattened or roundish, 

 uncolored bodies, whose work is usually to gather starch into 



Fig. I. — A cell (the megaspore) 

 from a lily ovule, filled with 

 l^-anular protoplasm, in which 

 IS embedded a large spherical 

 nucleus, containing a nucle- 

 olus, and accompanied by two 

 centrospheres, a. The line 

 around the protoplasm repre- 

 sents the cell-wall, with those 

 of the adjacent cells connected. 

 Magnified 500 diam. — After 

 Guignard. 



