8o Introduction to Botany. 



the thumb, and quickly thrust a glowing, but not blazing, 

 splinter into the tube. The splinter should blaze up, indi- 

 cating the presence of a large percentage of oxygen. 



91. Place a bell jar over any plant of proper size grow- 

 ing out of doors, having first fitted a piece of oilcloth, by 

 cutting a slit in it, closely around the base of the plant, 

 and having closed the slit with vaseline so that the mois- 

 ture from the ground will be prevented from rising into the 

 bell jar. Set another bell jar beside the first, on a piece 

 of oilcloth, but not over a plant. After a time compare 

 the amounts of moisture which have been condensed on the 

 inner surfaces of the jars. What does this teach as to the 

 transpiration of water from the leaves ? 



92. Mount a small piece of a leaf, with the under side 

 up, in chloral hydrate-iodine (see page 381), and examine 

 with a high power of the compound microscope. By care- 

 fully focusing on the upper surface of the mount, the sto- 

 mata (see page 86), or openings in the epidermis, can be 

 made out. They are made conspicuous by the starch in 

 their guard cells being stained blue by the iodine, while 

 the other cells of the epidermis are lacking in starch and 

 remain unstained. The opening between the guard cells 

 allows carbon dioxide to enter the leaf readily, and oxygen 

 to pass out. It also permits the water to evaporate from 

 the leaf. 



93. To determine whether the stomata are necessary 

 to the ingress of sufficient carbon dioxide for the manu- 

 facture of starch within the leaf. Select a plant, such as 

 the lilac, whose leaves have stomata on the under side 

 only. Keep a branch darkened for a day or two or until 

 the leaves are found destitute of starch by the method 

 described in Observation 89. Coat the under side of some 

 of the leaves with a melted mixture of equal parts of cocoa 



