105 
OBSERVATIONS FOR YOUNG BOTANISTS. 
VI. — The Functions of Leaves. 
J^^JSSIMILATION. — The first duty which a green leaf 
has to do is to take in the carbonic aciil gas from the 
RtwXl air. The presence of this gas has resulted from the 
expirations of all living creatures, from all sorts of 
combustion, and from chemical processes going on in volcanic 
regions, where large volumes of this suffocating gas are some- 
times poured out. It consists of one part of carbon (C) and two 
parts of oxygen (O) combined, and is represented thus (CO 2). 
This is a most refractory compound, and the only natural means 
known of decomposing it is the “ chlorophyll granules” of green 
plants. These consist of minute globules of living protoplasm 
coloured green. The greening is caused by light, as plants, like 
celery, grown without light, remain white or yellowish. 
Now the question is. What is \\\e force which pulls the C and 
O asunder ? It is light ; and if a beam of sunlight be made to 
pass through a three-cornered prism of glass, it is split up into 
all the coloured rays of the rainbow. This is called the solar 
spectrum ; and when light falls upon a green leaf, seven portions 
are stopped, a band in the red-orange, three very broad ones in 
the blue and violet end, and fainter ones elsewhere. So that 
if the leaf is thin enough to transmit any light, the green rays 
are the only ones visible, as also when reflected. This causes 
leaves to be green to the eye. 
Now it is by means of these bands of light that the living 
protoplasm can decompose CO2. That light can cause chemical 
changes is a familiar fact, for man}' coloured objects will fade if 
left in the sun, as carpets, curtains, &c. ; and we are all familiar 
with its effects on a prepared photographic plate. 
The effects produced in a green leaf are of the utmost con- 
sequence. The CO 2 is decomposed, the C is retained and Avith 
the aid of hydrogen (H) and O derived from water (HgO), a 
compound is made, which again, after undergoing certain 
changes, affords the first visible result, viz., starch (CgHj^^Og). 
This appears like Avhite specks on the green chlorophyll granules. 
They increase in size till they acquire the form characteristic of 
the plant. In potatoes they are large, oval, banded and with 
a dark spot. In rice they are very minute ; and so, for others. 
A simple way to prove the existence of starch is to take some 
very thin delicate leaf, as from the little aquatic star-wort, 
which has been exposed for some time to bright sunlight, and then 
with a camel’s hair brush sweep a little tincture of iodine over 
it. The leaf at once assumes a dark purple hue, as iodine has the 
effect of colouring the starch. If the leaf is thus tested after it 
has been in total darkness for some hours, then the iodine will 
fail to produce the result, showing that no starch is present. 
Starch is solid, and as such is of no use ; but at night it is 
