president’s address. 
593 
or as it is in the case of cuttings where from a similar callus roots 
will take their origin. 
Taking a tree as a plant of the highest development, we find it 
admirably adapted to all the requirements of its nature. The 
trunk firmly fixed to the soil by its roots, gives support to the 
branches, which divide up into numerous smaller ones, and these 
at their extremities give rise to leafy expansions, which are for the 
purpose of absorbing various gases from the atmosphere, and giving 
exit to watery vapour. The whole tree is encased in an outer 
covering, the bark, and in herbaceous plants by a thickened cuticle, 
both which are perfectly impervious to water, except at the grow- 
ing ends of the rootlets ; and the only parts that give entrance to 
air are the stomata on the leaves and the lenticels on the 
young growing shoots. 
It is in the green leaves of plants that all their food is 
manufactured, and the organs that carry on this work are 
the chlorophyll grains or chloroplasts, and the active agent 
is the chlorophyll. Those plants that have no green leaves, 
such as some parasites, saprophytes and fungi, obtain their food 
already formed, from their hosts, or from decaying vegetable or 
animal matter. The substance that forms the greatest proportion 
of the food of plants is carbon which is formed from the decom- 
position of carbonic acid of the air. It was formerly held that 
this gas in conjunction with salts and other substances obtained in 
solution from the roots, was assimilated at once by the protoplasm, 
the carbon being fixed and the oxygen set free ; but that is not 
exactly true. What takes place is this : — Under the influence of 
light, carbon dioxide and water are decomposed by the chlorophyll 
and a carbo-hydrate, — some form of sugar — is formed ; part of 
this becomes dissolved in the cell sap of the vacuoles, whilst 
a residuum is almost at once converted into starch grains, which 
remain in the chloroplasts a few hours as reserve material, for it all 
disappears during the night. At the same time oxygen is liberated 
and escapes by the stomata. Although carbon dioxide only exists 
to the amount of 1 or 2 per cent, in the atmosphere, yet from the 
enormous area of leaf suiface exposed and from the air being in 
