178 THE MICROSCOPE. Dec. 



Tradescautia A'irgiuica, fig. 6 from Water Crawf oot and fig. 

 7 from Hellel)ore. One may notice tlie difference in tlie 

 shape of the epidermis cells in the different plants, also 

 the different arrangement of the stomata; some are in rows 

 running lengthwise of the plant (fig. 2) and some are scat- 

 tered without order. In some cases they point invariably 

 in one direction (fig. 6), while in others they may point in 

 any direction (fig. 1). There is a great difference in the 

 number of the stomata. The under side of the oat leaf 

 has 2700 to a square centimetre, while on the under side 

 of the olive leaf there are 63,500 to the square centimetre. 



Each stomata consists of a pair of cells much smaller 

 than those of the epidermis generally and of quite a 

 difi"erent form. These two are called the guard cells. 

 Each guard cell is sausage-shaped and curved, the ends 

 of the cells being firmly joined together, while in the 

 middle they are separated a little, leaving an opening 

 between them which communicates with the intercellular 

 spaces. 



Xow make a section across the stomata lengthwise. 

 This may be done by holding a piece of the leaf between 

 two pieces of elder pith, or if the leaf is very delicate use 

 the pith of the sunflower. Make an incision in the pith 

 by laying it on something flat and hard and cutting 

 lengthwise of the pith. In this way it is less liable to be 

 broken. Then insert the leaf so it will come just above 

 the top of the pith ; hold it firmly together with the 

 fingers, or perhaps better, fasten with a rubber band and 

 cut a slice across both pith and leaf to make a smooth 

 surface. Now holding the pith so that the razor will cut 

 along the flat surface and not the edge, cut several very 

 thin sections and examine in water. You will probably 

 find some sections showing the stomata cut through the 

 center. This shows the two guard cells with the pore 

 between (figs. 5 and 8, s). The transverse of the guard- 

 cell is roughly square, the walls are very thick, especially 



