184 PRACTICAL LESSONS IN SCIENCE. 



cells are often thickened in various ways, forming wood cells, 

 spiral or banded cells, etc. 



New cells are formed by the division of old cells, each division 

 soon growing to the size and form of the original, as in the 

 growth of some of the lower forms of plant life, and in the tips 

 of growing roots. And new cells are also formed by the union of 

 the contents of two cells, as in the reproduction processes of some 

 of the lower plants. 



Besides the protoplasm within the cell wall there is often a 

 delicate green coloring matter called chlorophyll, which may be 

 dissolved out with alcohol. The chlorophyll gives the green 

 color to the leaves, stems and other parts of growing plants. 

 Another product of plant cells is starch, as in the potato, wheat, 

 corn and other grains. In addition to other substances many 

 cells contain solutions of various forms of sugar, as cane sugar, 

 grape sugaT, beet-root sugar, etc.; and the cells of some plants 

 secrete oils, as olive, castor, linseed and palm oil, the oils of tur- 

 pentine, lemon and thyme. Others secrete such substances as 

 camphor, essence of cinnamon, wintergreen, etc., and as the es- 

 sential oils of onions, mustard, asafcetida, etc. Others again 

 secrete gums, as gum tragacanth, gum arabic, and many others. 

 Several important acids are the product of vegetable cells, as 

 tartaric, citric and tannic acids. In addition to those mentioned, 

 opium, quinine, and other valuable medical substances, and many 

 valuable dyes are the products of cell action. 



The embryos of all plants begin as single cells, and some of 

 the lower plants during their whole existence consist only of 

 single cells. In some cases cells at first distinct become united 

 more or less closely into a common mass, each cell to a great ex- 

 tent remaining a distinct individual. Sometimes the partitions 

 between contiguous cells are dissolved away, and the two cells 

 are merged into one. In this way long tubes or vessels are 

 formed. But in the great majority of plants the cells are com- 

 bined into tissues. They may be joined end to end, form- 

 ing threads, filaments or hairs, or they may be joined end- 

 wise and sidewise, forming cell surfaces, or they may be joined 



