THE LEAF 125 



epidermis, leading into the leaf from the outside air ; 

 they are called stomata. Their number is enormous. 

 One lime leaf contained more than a million of them ; 

 and this number need excite no dismay, because the 

 method of reckoning the stomata is very simple and 

 precise. The restriction of these organs mainly to the 

 lower surface of leaves explains why it is that, in experi- 

 ments such as the one described above, gas is observed 

 to be given off in most cases on the lower surface of the 

 leaves. We shall return later on to these stomata. 



Let us see what kind of gas is given off by a leaf when 

 sunlight acts upon it. For this purpose we must wait 

 until a sufficient quantity of gas is accumulated under 

 the glass bell (fig. 34) . Then we take out the cork and 

 introduce a slightly smouldering splinter into the neck of 

 the bell. It instantly glows and bursts into flame, 

 scattering sparks in all directions. This is the regular 

 test for oxygen. Therefore the air given off by leaves 

 is oxygen, or a gas very rich in oxygen. We have 

 already seen that this experiment is successful only when 

 carbonic acid is dissolved in the water. Is there not 

 some connection then between the presence of carbonic 

 acid and the formation of oxygen ? Evidently our 

 experiment does not give a sufficient answer to the 

 question. In order to find out what happens to the 

 carbonic acid, we make another experiment. Let us 

 take an apparatus consisting of a tube in the shape of 

 a horse-shoe (fig. 36), with one side closed and the 

 other with a stopper. 1 We pour some water into the 

 tube and introduce carbonic acid in such a way as to 

 fill the space in the left-hand closed end of the tube up 

 to the movable pointer on the stand (as is shown on 

 the figure) . We lower the long leaf of a cereal into the 

 right-hand open end of the tube, and having filled the 

 tube up to the very top with water close it with the 



1 Known as Hofmann's apparatus, very much used in the practical 

 teaching of chemistry. 



