594 
PRESIDENTS ADDRESS. 
constant motion, a sufficient supply is always obtained. As starch 
granules are always found in the chloroplasts, it was thought that 
that substance was the first product of carbon dioxide decomposi- 
tion, but subsequent research has proved this not to be the case. 
Whenever starch is found in a plant it is as reserve material, and 
before it can be utilised, the grains being insoluble and consequently 
unable to pass through cell walls, it is reconverted into some form 
of glucose and in this state is able to be carried in solution to all 
parts of the plant. An example of this is seen in the potato, 
where the starch that is formed in the leaves is converted back 
into glucose which is then carried by the stem to the tubers, where 
it is again reconverted into starch, and is stored up for the future 
use of the young plant. With regard to the nitrogenous substances, 
it is not at present known exactly how or where they are formed. 
Probably the first steps in the process commence under the in- 
fluence of the chloroplasts, and then in conjunction with nitrates 
absorbed from the soil, further elaboration takes place in other 
parts. The formed material is conveyed by osmosis from the leaf 
through the parenchyma of the petiole to the cambium district, 
i.e., between the bark and the wood, whence it is distributed 
as food throughout the plant ; some of it to be utilised, whilst some 
is conveyed away as reserve material to be stored in roots, winter 
buds, seeds, &e. The assimilation of carbon dioxide is carried on 
solely in the chlorophyll apparatus of the leaves, the chloroplasts 
are the laboratories and the chlorophyll is the active agent which 
under the influence of light brings about the decomposition. The 
energy therefore made use of is derived direct from the sun, but 
photosynthesis is brought to a standstill if the light is too bright, 
for the action is carried on much more actively in subdued light. 
Up to the present it has been found impossible to ascertain the 
composition of chlorophyll, on account of its great readiness to 
break up in any attempt to isolate it. The chloroplasts owe their 
green colour to the presence of iron, for if this metal is not supplied 
to the plant the chloroplasts are not coloured, and if they are not 
coloured, they are unable to effect the decomposition of carbon 
dioxide ; and yet iron does not enter into the molecule of the 
