MICHIGAN ACADEMY OF SCIENCE. 
49 
the under sides of leaves and of the surfaces of young steins and roots, 
is due quite largely to the presence of air in the intercellular spaces. 
If the intercellular spaces of a leaf be injected with water, the leaf 
loses its whitish appearance, and soon dies owing to the lack of oxygen. 
These air spaces in a leaf have intercommunication with one another, 
and a direct communication with the openings of the stomata; so that 
the aerating system of a leaf is a continuous one. The openings of the 
stomata are very numerous, sometimes as many as 250 per square mm. 
The aerating system is best developed in leaves, probably because of the 
great vital activities carried on there. Aquatic plants have their stomata 
and intercellular spaces of leaves on the upper sides of the leaves, and 
it is due to the air in these intercellular spaces that the leaves float. 
The materials in the tissues are actually heavier than water, and con¬ 
sequently would sink were it not for the buoyancy brought about by 
the presence of air among the cells. Stomata occur on green stems and 
in structure and operation are similar to those on leaves. 
Lenticels, which are eruptions through the outer cortex of branches, 
originate from stomata, but become much larger, and many cells are 
involved as growth of stem enlarges them. They serve the function of 
interchange of gases, but in one respect they are better adapted to this 
than are the stomata. The outer cells are corky and therefore slightly 
permeable to air, but not to water, consequently the mycelium of a 
fungus can not enter as it could through the openings of the stomata. 
As the lenticel enlarges, new cells are formed by the cork cambium be¬ 
neath, and those new cells become corky later, partly through the action 
of the air. If the amount of corky tissue be very great it becomes 
almost impermeable to gases, but as growth proceeds, the lenticel breaks 
open the corky layer and this renews the communication. The cork 
cambium forms new corky cells beneath. 
On the roots of some bog plants, an outside tissue similar to that of 
the lenticel, excepting that it covers a much greater area, serves a func¬ 
tion similar to that of the lenticel. It permits the passage of gases 
but not water. This tissue is called aerenchvma. 
As roots have no stomata and few lenticels, air must pass in through 
the epidermal structures directly. This is particularly the case with 
all young growing roots, some of which are capable of gaining sufficient 
oxygen directly through the epedermal cells from water. But the great 
majority of land plants are not able to do this. The roots of such plants 
must obtain oxygen from the air in the soil; and young vigorous roots 
require a very considerable amount. 
Air in wood cells and vessels, in pith and the like, may or may not be 
in a position to furnish a supply of oxygen to the living cells. However, 
it seems to serve the function of buoyancy and strength, and indirectly 
connects the aerating systems of roots, stems and leaves. 
There can be no question then as to the importance of a knowledge 
of the aerating systems of plants. The whole subject of drainage is one 
directly involved. The presence of water sufficient to cover the roots 
of plants for any length of time, is death to such roots, owing to the 
supply of Oxygen being cut off. The question of soil tillage is one quite 
largely of rendering the soil thoroughly permeable to air. It is all the 
more important to have soil in such condition because the total amount 
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