PLANTS OF THE INDIAN DESERT. 105
The pericycle in both the species of Aerua consists of long thin
groups of stone-cells. In the rhizomatous axis of A. tomentosa it
forms a composite ring of stone-cells except at certain spots where
stone-cells are not developed and where the space is occupied by the
conjunctive tissue of medullary rays, which is very extensive where
the gaps are large. Sometimes lenticels are found above these gaps ;
it may therefore be supposed that this conjunctive tissue forms a
system of communication between the lenticels and the vascular
tissue.
In P. lappacea the stone-cell groups of the pericycle are trian- gular. The bases of these groups on two opposite sides of the axis are lengthened out, so that adjacent groups unite and form composite strands, while on the other two sides the stone-cell groups are isolated, the pericycle thus presenting an isobilateral symmetry. In A. aspera a similar isobilateral arrangement is presented, but in a different way. The stone-cell groups on two opposite sides of the axis are longer and thinner than those on the other two. The isobilateral symmetry of pericycle may have been developed by the direction of the prevailing wind, the more composite structure being developed in the plane more affected by the wind.
The wood forms a composite ring. The interfascicular wood prosenchyma is much more extensive and is composed of cells with much more thickened walls and with much smaller lumen, in both the species of Aerua than in the case of A. aspera and P. lappacea. In P. lappacea the arrangement of xylem bundles is isobilateral, larger bundles being developed below the composite stone-cells strands and smaller ones below isolated stone-cell groups. This suggests that functional activity is greater on these sides which are more affected by the wind. In A. aspera there occur two vascular bundles in the pith with soft bast on their outer side and placed in the plane in which stone-cell groups form composite strands.
Soft bast forms groups on the outer side of xylem bundles in both the species of Aerua, while in A. aspera and P. lappacea it forms a continuous ring.
Pith consists of thick-walled cells in species of Aerua; in A. aspera and P. lappacea it is formed of large thin-walled cells.
The rhizomatous axis of A. tomentosa presents many anamolous structures. Wood forms a composite ring and is composed of a few xylem bundles with very large vessels. Medullary rays are unusually very broad ; the broadest ones broaden outwards in the form of wedges and come into contact with the cork tissue. In addition to the outer wood-ring there is a central wood cylinder developed in the pith tissue. It is composed of four diagonally placed xylem bundles, two being
