HOW PLANTS LIVE AND GROW 27 
we now require a third gas, nitrogen. This is present in 
the air, but plants never get it from that source, always 
preferring to draw it up in compounds from the roots. 
The chief object of manuring is to ensure that there shall 
be plenty of these nitrogen compounds in the soil. With 
these three gases, carbon, and a few traces of minerals, 
‘especially sulphur, potash, and phosphorus, the proto- 
plasm can keep itself going, and also form fresh reserves 
for the future in the shape of the alewrone-grains, which 
correspond to the starch grains for the cell-walls. 
There is one curious point about the chlorophyll 
corpuscles which is worth remembering. They only work 
in the sunlight, and therefore at night plants no longer 
are giving off their spare oxygen. In the daytime, though 
they are always breathing oxygen, they are giving off far 
more than they are taking in, but at night they are 
simply using it up, and there is for this reason some 
ground for the belief that plants are not good in a 
bedroom. 
All this work of the leaves is so important that you 
can see what a disaster it is for plants to lose them. A 
late frost in spring may nip any number of leaves and 
kill them, and the tree’s growth for the year is heavily 
handicapped. Other leaves that were too closely in bud 
for the frost to get at them may enable it just to keep 
alive, but the cambium cells will be checked in their 
splitting, and very little new wood comparatively will be 
made. If it be a small plant with all its leaves unfolded, 
the failure of the starch department will often lead to 
its entire collapse and death. You might bear this in 
mind when next you feel inclined to pull the leaves off 
- any bush by the wayside. It can give you small pleasure, 
and it handicaps the plant very seriously. 
