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lie outside the wood and are spoken of as the bast-tubes. 
These are very delicate, and to prevent them from being 
crushed they are protected on the outside by strong resist- 
ant fibres called the bast fibres or hard bast. : 
With regard to the essential work of leaves their 
primary function is without.doubt to build up organic 
food material from the water taken up by the roots, This 
they are able to combine with carbon which they take 
from the atmosphere, where it occurs plentifully in the 
form of carbonic acid, the gas we breathe out from our 
lungs. In the presence of sunlight all parts of the plant 
which contain the characteristic green colouring matter are 
able to break up the carbonic acid, retaining the carbon 
and giving back the oxygen which was previously com- 
bined with the carbon. In doing this they are constantly 
ere as it were the air and enriching it with the 
ife-sustaining oxygen needed by man and all forms of 
animal life. The carbon which the plants retain is com- 
bined with the elements of water to form organic material 
called carbohydrates, of which starch and sugar are the 
most important to plants. 
Within certain limits the brighter the light the more 
active is this process of carbon nutrition, or assimilation 
as it is called. Hence the great importance of giving 
plants as bright and sunny a position as possible, and 
hence also the reason why plants seek to secure the 
most favourable positions for their leaves. The formation 
of leaf mosaics, mentioned above, is a good instance of 
means adopted to secure the most effective display of 
leaves. Unless specially adapted for growing in shade, 
as are for instance most ferns, the majority of plants will 
not show their best development unless grown in an open 
situation. Apart from the essential influence of light upon 
the process of leaf nutrition, it has been found. by experi- 
ment that this important nutritive process is affected by 
temperature being within certain limits proportional to 
the rise in temperature. This explains why in sheltered 
places, where the air is heated up in winter many plants 
are known to make better growth than actually in the 
open. Shade can be partially counteracted by warmth. 
In frames in which plants often get less light than in the 
open, the warmer temperature enables them to grow more 
rapidly than were they exposed to the colder air. It 
must not be forgotten too, that not only is the process of 
