176 Leaves and their Work 



barley, for instance, from which malt is made, gives off 

 quite a high degree of heat. 



As the plant, like the animal, must breathe in order 

 to live, it must needs have a constant supply of air for 

 breathing, as well as feeding, purposes. And this air is 

 necessary, more or less, not only for leaves and blos- 

 soms, but for all parts, including stems and roots. It 

 breathes through all, though chiefly by its leaves and 

 blossoms ; and the roots are always breathing out 

 carbon-dioxide into the soil. If the soil be too close, 

 or baked on the top, the roots cannot get enough air to 

 breathe properly, and the plant is sickly. And the 

 same thing happens when soil is piled up round the 

 stem, for the air is then kept away both from it and 

 from the roots, and many a tree has been killed in 

 consequence. 



Leaves covered with dust, or smoke-particles, cannot 

 possibly thrive, for they are choked, just as much as an 

 animal would be if kept without air ; and besides being 

 unable to breathe, they are also unable to take in food. 

 The whole plant, therefore, suffers, and unless relief 

 came from time to time in the shape of rain to wash 

 the leaves, it must die of suffocation. 



The patient, laborious Chinese, who were the first to 

 attempt market-gardening in the sandy environs of 

 Melbourne, were so well aware of the fact that plants 

 cannot thrive when choked with dust or sand, that they 

 were in the habit of washing their vegetables, leaf by 

 leaf, three times every day to keep them in health. 



