24 THE GLORY OF THE GARDEN 



plants to breathe out as much CO2 as is emitted 

 by one human being, surely a few plants or flowers 

 are not such dangerous room-mates as one human 

 being ! 



Scientists tell us that, if it were not for the fact of 

 plants purifying the atmosphere by breathing in 

 the poisonous CO2 and giving out fresh oxygen, 

 the earth would soon be quite uninhabitable; we 

 should have such a superabundance of impure 

 gases hanging on the air unused, and to us un- 

 usable without the aid of plants, that we should 

 shortly be poisoned with these gases, and the earth 

 would become a dead planet. 



l^he Fall of the Leaf. — In very hot countries 

 trees retain most of their leaves all the year round; 

 a leaf falls occasionally, but no considerable portion 

 of them drop at any one time. This holds good 

 with regard to our own pine, fir, and spruce trees. 

 The impossibility of absorbing moisture from the 

 soil when the ground is very cold and near freezing- 

 point would cause the death by drying up of trees 

 with broad leaf surfaces. Also, in countries where 

 there is much snow, most large-leaved trees could 

 not escape injury to their branches from over- 

 loading with snow. For these reasons most of 

 our forest trees are deciduous — that is, they shed 

 their leaves at the approach of winter, and the 

 tree remains in a dormant condition. 



In the autumn the green pigment in the leaf 

 breaks up, leaving behind yellow granules, and these 

 produce the autumn colour. This brilliant coloura- 

 tion of autumn leaves is often quite wrongly sup- 

 posed to be due to the action of frost. 



But it depends merely on the changes in the 



