PRODUCTION OF AIR PASSAGES IN THE ROOT OF ZEA MAYS. 135 
Ordinary Soil. — This is a favourable soil, but is slightly 
inferior, as regards aeration, to the potting soil. 
Various Water Cultures. — The media were boiled, and then 
there was very little air obtainable by the roots ; the food 
supply was good, all the necessary salts being present. 
Variations in the Structure of the Roots. 
Taking those roots developed in Potting Soil as a normal 
type, we find externally no feature of any marked interest. 
The roots at the stage examined were the usual type of 
slender cylindrical ones, possessing a fairly plentiful supply 
of root hairs. 
Internally, the central cylinder is of the usual type, and 
shows nothing abnormal in its development. Surrounding 
this central cylinder there is a cortex about six layers of 
cells in thickness. It is in this layer that the greatest 
variation occurs. 
All through the series of roots there was a distinct 
tendency either to form or suppress the air-spaces, and 
these air-spaces seemed to show a relation to the external 
conditions. In the case of the potting soil these air-spaces 
were but scantily developed. The younger and central 
portions of the root were free from them, and it was only 
in the upper portions that they were found to occur, and 
even here they were not always present. 
In every case a number ot sections were made in the 
various portions of several roots, and the results obtained 
based on the general distribution and the general occurrence 
of these air-spaces. 
Sand. — These roots were of the general type ; they were if 
anything inclined to be rather swollen in appearance. The 
central cylinder was well developed. The cortex seemed 
absolutely free from air-spaces ; none of the sections from 
the upper, middle, or lower portions showing any trace of 
them. This was the only condition under which the plant 
was absolutely free from them. Fig. i is a drawing of a 
typical section of the upper portion of a root grown in sand. 
Damp Sawdust. — The development of root hairs was very 
scanty, most roots being practically without them. 
The central cylinder is well developed, more so than in 
the potting soil. 
The air-spaces occur throughout the roots ; they appear 
to be formed by the breaking down of groups of cells, the 
cell walls in many cases are not completely broken and 
stretch across the air-spaces. 
Fig. 2 is a drawing of a typical section. 
Damp Clay Soil. — There was a fairly plentiful development 
of root hairs ; the central cylinder was also well developed. 
The air-spaces were generally conspicuous throughout the 
roots, appearing in both the older and younger portions. 
They formed a fairly regular system round the central 
