144: VEGETABLE FABKIC. [lESSON 23. 



soft and yielding, as young cells are, when pressed together they will 

 become twelve-sided, like that in Fig. 339. And a section in any 

 direction wiU be six-sided, as are the meshes in Fig. 340. 



398. The size of the common cells of plants varies from about 

 the thirtieth to the thousandth of an inch in diameter. An ordinary 

 size is from ^^ij to -^^u of an inch ; so that there may generally be 

 from 27 to 125 millions of cells in the compass of a cubic inch ! 



399. Now when it is remembered that many stems shoot up at 

 the rate of an inch or two a day, and sometimes of three or four 

 inches, knowing the size of the cells, we may form some conception 

 of the rapidity of their formation. The giant Puff-ball has been 

 known to enlarge from an inch or so to nearly a foot in -diameter 

 in a single night ; but much of this is probably owing to expansion. 

 "We take therefore a more decisive, but equally extraordinary case, 

 in the huge flowering stem of the Century-Plant. After waiting 

 many years, or even for a century, to gather strength and materials 

 for the effort, Century-Plants in our conservatories send up a flow- 

 ering stalk, which grows day after day at the rate of a foot in twenty- 

 four hours, and becomes about six inches in diameter. This, sup- 

 posing the cells to average ^^^ of an inch in diameter, requires the 

 formation of over twenty thousand millions of cells in a day ! 



400. The walls of the cells are almost always colorless. The 

 green color of leaves and young bark, and all the brilliant hues of 

 flowers, are due to the contents of the cells, seen through their more 

 or less transparent walls. 



401. At first the walls are always very thin. In all soft parts 

 they remain so ; but in other cases they thicken on the inside and 

 harden, as we see in the stone of stone-fruits, and in all hard wood 

 (Fig. 345) Sometimes this thickening continues until the cell is 

 nearly filled up solid. 



402. The walls of cells are perfectly closed and whole, at least in 

 all young and living cells. Those with thickened walls have thin 

 places, indeed ; but there are no holes opening from one cell into 

 another. And yet through these closed cells the sap and all the 

 juices are conveyed from one end of the plant to the other. 



403. Vegetable cells may vary widely in shape, particularly when 

 not combined into a tissue or solid fabric. The hairs of plants, for 

 example, are cells drawn out into tubes, or are composed of a row 

 of cells, growing on the surface. Cotton consists of simple long hairs 

 on the coat of the seed ; and these hairs are single cells. The hair- 



