48 
EIGHTH REPORT. 
THE AERATING SYSTEMS IN PLANT TISSUES. 
,T. B. DANDENO. 
For all living plants oxygen is an essential substance. For green 
plants both oxygen and carbon dioxide are essential. So that the rela¬ 
tion of plants' to these two gases, is an important one. Though the 
final word has not yet been said concerning all the details of the rela- 
townships, yet considerable advance has been made towards a knowledge 
of the underlying principles within the last ten years. 
Oxygen is necessary for the activity of the protoplasm in the living 
cell. When plants are dormant, very little oxygen is required; when 
they are growing rapidly, a very considerable amount is necessary. The 
amount of oxygen necessary for some of the lower forms of plants is 
surprisingly large. For instance—man inhales about .4 per cent of his 
body weight in twenty-four hours of oxygen; certain molds consume 2.2 
per cent of their body weight of oxygen; active bacteria 72 per cent of 
their body weight. And as a general rule, the volume of oxygen con¬ 
sumed is equal to the volume of carbon dioxide set free. The process is 
known as respiration. 
In living cells, in the presence of chlorophyll, sunlight and water, car¬ 
bon dioxide may be built up into carbohydrates. If _ respiration be 
going on in a green cell at the same time, the carbon dioxide liberated 
is, under normal conditions, utilized in the formation of carbohydrates. 
This utilization of carbon dioxide in the process of formation of starch 
or other carbohydrate, is termed photosynthesis. Both these processes 
may be carried on at the same time in the same living cell, if all condi¬ 
tions be favorable. 
For regulating the interchange of these two gases, several special 
plant structures have been developed. The more important of such 
structures are:—stomata, lenticels, aerenchyma, unmodified epidermel 
cells, empty dead cells, and intercellular spaces. Stomata may occur 
on leaves or branches but never on roots. Lenticels are found on 
branches and sometimes on roots. Aerenchyma is a peculiar tissue de¬ 
veloped on the surface of the roots of certain plants which grow in mud. 
Epidermel cells, if not cutinized or suberized, are permeable to gases 
in whatever part they may occur. Intercellular spaces are common in 
almost every tissue of the plant. Empty dead cells, such as occur in 
pith and wood, always contain more or less air. 
The openings of the stomata are extremely small—about 1,000 
of them being required to cover an opening whose area is one square 
mm. These, of course, are too small to allow dust particles, or water 
in a liquid state, to pass, unless under high pressure. Gases pass only 
very slowly. The force which causes the passage of gases, either in¬ 
ward or outward, is the vital activity within the living cells coupled 
with ordinary atmospheric pressure. The white velvety appearance of 
