2O 



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 

 purifying as it were the air and enriching it with the 

 life-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 



