96 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
The different effects of the glasses are seen very obviously in the growth 
of the plants. Thus long weak stems are produced under red, yellow, and 
green glasses, but short ones under blue and violet. Again, the amount 
of water retained in the tissues, or its degree of succulency, is much 
greater under the red end of the spectrum than under the more refrangible 
rays. 
The conclusion is that all coloured glasses are, separately, injurious to 
plants, the worst of all being green, as it arrests so much of the light 
right and left of it in the spectrum, which is absorbed by chlorophyll, 
i.e. just those rays which are mainly required for these two functions. 
With regard to the results of assimilation the first process is the 
decomposition of carbonic acid (CO2) in the air by the green-coloured 
protoplasm of the chlorophyll granules. The various chemical substances 
formed in succession are not perfectly known, but the first visible product 
is starch. This appears as minute granules on the chlorophyll grains. 
They increase in size until the latter may disappear from view. 
As soon as light has gone the starch is converted into sugar, and this, 
being soluble, can be conveyed away to the growing parts, there to furnish 
material for building up cell-tissue. If there is a superabundance at 
the end of the growing season, it is stored up as starch or oil in seeds, 
tubers, cellular parts of stems, &c., for future use. 
It is worth while observing that no other known method exists in 
nature by which CO2 can be decomposed, than by protoplasmic chlorophyll 
granules ; though CO2 is constantly being formed by respiration, 
combustion, and other chemical processes, and the oxygen of the air cor- 
respondingly utilised. This element, however, is restored to the atmo- 
sphere whenever COo is decomposed by the green parts of plants. 
If a chemist wished to decompose CO2 he must find some substance with 
a more powerful affinity for oxygen than carbon. He therefore takes a 
piece of the metal sodium, placing this in a flask full of CO2 and 
applying heat from a lamp ; for it will do nothing at an ordinary tempera- 
ture. The sodium now seizes upon the oxygen, and forms caustic soda, 
while the carbon is set free as a black powder. 
But the chemist can go no further. Chlorophyll granules decompose 
the CO2 without these violent chemical methods, and then instantly 
utilises the carbon set free by combining it with the elements of water ; 
and so in time makes starch, the composition of which is C^HioOr,, i.e. 
really six atoms of carbon (C) united with five molecules of water (H2O). 
It has been thought that CO2 might be supplied to plants dissolved in 
water taken up by the roots, but all experiments give negative results. 
Indeed it would appear that no green part can decompose CO2 unless it 
be actually presented to it directly from the air. It cannot even be trans- 
mitted from one half of a leaf to the other half ; for if a leaf be half 
under a bell-glass containing CO2 in the air, while the other half is under 
a glass in which there is none, it is only in the portion exposed to CO2 
where starch will be found, though the whole is equally illuminated. 
With regard to the other function, viz. transpiration, or the exhalation 
of superfluous water from the green parts of plants, this too is effected 
by the agency of special rays of the spectrum. The use of it is that the 
plant may imbibe sufficient mineral matters from the soil. As these are 
