FACTORS INFLUENCING PHOTOSYNTHESIS 259 
combustion, and all oxidation processes in maintaining the supply 
of carbon dioxide for photosynthesis. Roughly estimated, 150 
square meters of leaf area will use up in one summer all of the 
carbon dioxide which an average man produces through respira- 
tion in one year. 
When one considers that the amount of carbon dioxide in the 
air is only about 0.03 per cent, that is, about 
3 parts in 10,000 parts of air, it is surpris- 
ing that plants can make sugar as rapidly 
as they do. Sometimes, as around cities 
with many factories, the per cent of carbon 
dioxide may be a little higher but it is 
always exceedingly low. Of course carbon 
dioxide is present in solution in the soil 
water; but it is easily demonstrated that 
this carbon dioxide is of practically no help 
to plants in photosynthesis. To compensate 
for the limited amount of carbon dioxide, 
it is obvious that leaves need broad surfaces 
and a thorough distribution of chlorophyll, 
so that their absorbing surface may be 
large. However, with all of these adjust- 
ments of the plant, it has been demonstrated 
Fig. 236.— Leaf, 
that the normal supply of carbon dioxide 
is often insufficient for the maximum 
amount of photosynthesis; for some plants, 
when surrounded by air in which the 
amount of carbon dioxide is increased up 
to 1 per cent, show a corresponding rise in 
photosynthetic activity. 
Since stomata are the openings through 
which carbon dioxide enters the leaf, their 
showing the effect on 
photosynthesis of clos- 
ing the stomata. The 
stomata on the under 
surface of the white area 
were closed by covering 
the epidermis with vase- 
line, thus filling the 
stomata and excluding 
carbon dioxide. After 
Nadin. 
number per area of leaf surface and the aoa 
extent to which they are open affect the amount of this gas that 
reaches the mesophyll. That photosynthesis is inhibited when 
stomata are closed is demonstrated by the experiment shown in 
Figure 236. The experiment shows the necessity of keeping the 
stomata free from dust and other bodies, such as spores of plants . 
and secretions of insects, that clog the stomatal openings. It is 
for this reason that’ we are advised to cover house plants with a 
