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. 
