414 Transactions of the Royal Society oj South Africa. 
ness. In higher-land plants the living cells are generally so arranged that 
small places are left between them. These spaces are filled with air and 
are connected with each other and with the exterior by special con- 
trivances in different parts of the plant. In the leaf minute pores are 
found in the outside skin through which air percolates into the leaf tissues 
and so to the more remote intercellular spaces. In thicker stems, how- 
ever, larger openings are actually formed in the bark, and loose tissue is 
exposed to the air through which air can also percolate to the more 
internal intercellular spaces. The pores in the leaves are known as stomata, 
and the openings in stems are known as lenticels. In very thick trunks, 
however, where there is a thick bark, it is not so apparent how air obtains 
an entrance. It must be noted, however, that in a trunk living cells are 
only found as a fairly thin layer just inside the bark, the centre of the tree 
consisting of dead and empty cells. It is quite probable that the living 
cells being close to the water-conducting tissue obtain their supply of 
oxygen from the water as it passes upwards. A gum-tree has a closely 
set bark of at least an inch in thickness, in which there are no lenticels. 
The living cells of roots are dependent on the air found between the 
particles of soil, except in the case of thick roots, where the water current 
may be again utilised. However, some trees which live in water-logged 
soil and deoxygenated mud send up a special kind of branch from the 
roots into the air provided with special aerating tissue. Such roots are 
called respiratory roots, an example of which is found in the Mangrove. 
In water plants with deep-seated tissues large spaces are always found 
filled with air. These, of course, may also be of service in keeping the 
plants more or less erect in the water. Some terrestrial plants too, such as 
grasses, have hollow stems. Although this may serve as an aid in respira- 
tion it is probable that this structure is more for the purpose of economy 
in building. 
In animals a new method of aeration of the deep-seated tissues now 
appears. That is by a liquid which can carry in it more oxygen than can 
be carried by water. In the simplest kind of animals using such a means 
this liquid is found permeating the tissues and filling large irregular spaces 
in the tissues. An advance on this is found in animals in which the liquid 
is conveyed to all parts of the body by a ramifying system of tubes, so that 
eventually each cell comes in contact with this oxygenated liquid. The 
oxygen in the liquid, of course, comes from the air, and it now becomes a 
question of keeping up a constant supply of oxygen or air to this liquid. 
The whole secret of this liquid carrying oxygen about lies in the fact that 
it contains a peculiar chemical crystalline compound either dissolved in it 
or contained in tiny special cells floating about in it. This substance is 
hsemoglobin, and it has the property of easily combining with oxygen and 
as easily giving it up again to the cells with which it comes in contact. 
