54 



of frost, we find that with a lowering of temperature some 

 of the water from the cell-sap passes into the spaces 

 which exist between the cells in mature tissues. Thus the 

 sap becomes more concentrated and less liable to injury, 

 while the water which may become frozen in the inter- 

 cellular spaces does no damage if the latter are of fair 

 size. If they are small, then the expansion of the water 

 in freezing may tear the tissues asunder and thus injure 

 them. In the growing parts of the plant there are tiny 

 intercellular spaces, so that little water can be passed 

 out of the cells, and consequently the sap itself may be 

 frozen, in which case the protoplasm, the living substance 

 of the cells, is killed and the tissues become blackened. 

 A very sudden frost is always more harmful than a long 

 spell of frost preceded by a gradual lowering of the 

 atmospheric temperature, as plants are able to prepare 

 themselves by a condensation of their cell sap. It is the 

 same with recovery from frost. Plants in a frozen con- 

 dition may recover readily if they are slowly thawed. 

 When exposed to bright sunshine on a frosty morning they 

 may be permanently injured as often happens with Wall- 

 flowers. It is therefore well to shade frozen plants from 

 the direct rays of the sun, so that they are thawed less 

 rapidly. We must remember that in bright sunshine the 

 pores of the leaves open and the plants are rapidly trans- 

 piring, and if this takes place while the ground is still 

 frozen and the roots are unable to absorb water, the con- 

 sequences may be serious. 



Those of us who have laboured in town gardens are 

 only too familiar with disappointment due to general sick- 

 liness of some of our plants and to the harmful effect 

 of a smoky atmosphere. Worse in winter than in summer 

 it is nevertheless pronounced even in the latter, and records 

 taken show that there are quite a number of gloomy days 

 in July and August, and that air pollution is quite appre- 

 ciable during these months. From actual measurements 

 made during the month of July, it has been calculated 

 that if we draw a circle of a mile radius around the Town 

 Hall of Manchester, 195 tons of impurities would be col- 

 lected during the month from this area, and of this 

 100 tons would consist of soot or other insoluble matter;* 

 * See First Annual Report of the Sanitary Committee of the City 



of Manchester on " The Work of the Air Pollution Advisory 



Board." March, 1915. 



