t68 Leaves and their Work 



rells of a honeycomb, except that they vary extremely 

 in size and shape. 



Some cells are so minute as to be altogether invisible 

 to the naked eye; as, for instance, the spores of 

 lichens and fijngi, the ' germs ' already mentioned, and 

 certain minute water plants, each and all of which 

 consist of a single cell, filled, like the honey-cells, with 

 more or less fluid contents. 



But the cells in the flesh of a lemon are gigantic by 

 comparison, being half an inch long; the cells of 



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Transverse section of a leaf, magnified. 



fibres are often much longer than this, and there are 

 cells of all sizes between these, the most usual size 

 being from -yt^js to ^^ of an inch across. Cells are of 

 almost every possible shape, too — globular, square, 

 six-sided, twelve-sided, or quite irregular, with their 

 outlines beautifully zigzagged or waved. But, whatever 

 their shape and size, their walls, thick or thin, are 

 composed of the skeleton material mentioned above, 

 which is called after them ' cellulose.' 



This material, like cotton and other fibres, is com- 

 posed in great measure of carbon, and, as all plants in- 



