50 
EIGHTH REPORT. 
of air in the neighborhood of the roots is necessarily limited, owing to 
the fact that the space is largely occupied by soil. And if decomposing 
organic matter be present, a considerable amount of carbon dioxide will 
be liberated, and this gas, if abundant, may be quite injurious to roots. 
Moreover, decomposing matter in the soil always draws upon the sup¬ 
ply of oxygen there, so that less is available for the roots. Soil is 
liable to become baked, and, in the case of grass fields, to be so coated 
over as to reduce very materially the supply of available air to the roots. 
Old pastures ought to be harrowed up. Orchards should not remain 
sodded. Top crusts should be broken. And the fact should not be 
lost sight of that young roots are parts which grow rapidly and conse¬ 
quently need plenty of oxygen. There is no question that plants differ 
widely in regard to their power of adapting themselves to unfavorable 
aeration of soil, but the principle holds rigidly throughout. And what 
is true of roots is true also of other underground parts—stems and 
leaves. 
Concerning the part of the plant above ground, the matter seems of 
less consequence only because the air supply is self-regulative, owing 
to the fact that it is abundant at all times, and it is usually accessible 
without any help from man. The parts above ground have, therefore, 
a great advantage over the underground structures with respect to the 
aerating system. Moreover, the air is constantly in motion and the 
plant never suffers from lack of change of air. The motion itself is of 
advantage in bringing about a current of air which continually bathes 
the external aerating organs. 
It is obvious, however, that if any interference be made with the 
special aerating system, or the aerating organs, it will result in injury 
to the plants. Some people smear over whole twigs, branches, and 
even trunks of trees with a greasy wash, not knowing that they are 
choking up the air passages and injuring the trees. 
Although lenticels are usually not visible on old stems of trees, yet 
eruptions, much larger than lenticels, are being continually formed"as 
the tree increases in size. The cork cambium is ever present to heal the 
wound as soon as a break occurs, and such eruptions or cracks are 
often quite large. New eruptions serve as lenticels. On smooth-barked 
trees, such as poplar, they may be readily seen, a single one often as 
large as a silver dollar. These form part of the aerating system of the 
tree trunk, and the air which enters at these places is near the growing 
tissue in the cortex, just where it is needed. And it has been proved 
that carbon dioxide is given out at these places. The heart-wood cells 
are wholly filled with air, and the sap wood contains more or less, con¬ 
sequently there is air both within and without the growing tissues. 
The air in the wood cells is, however, not freely available for use in 
growth. It serves mainly the purpose of buoyancy and strength for 
the stem. 
The leaves being the most active organs of the plant, have conse¬ 
quently, the best aerating system, both as to means by which air may 
pass in or out, and as to aeration within the tissue. The stomata are 
not the only means. In many cases the epidermal cell walls permit 
an interchange of gases. If the stomata be clogged up with grease, the 
plant suffers. If the whole surface be smeared over, the leaf dies. If a 
