104 THE JOURNAL OF INDIAN BOTANY.
large clustered crystals in the neighbourhood of veins of all members and in the cortex and pith of both species of Aerua and in A. aspera. In the rhizomatous axis of A. tomentosa numerous clustered crystals are present in the neighbourhood of water-storing tracheids, in many of the medullary ray cells and in the pith. In the axis of P. lappacea oxalate of lime is found in the form of crystal sand composed of triangular granules in the cortex and pith.
The veins are embedded except those of the mid-rib which are vertically transcurrent above and below by collenchyma. They are enclosed in bundle-sheaths of thick-walled green cubical cells in both the species of Aerua. The mid-rib is furrowed above and prominent below in A. pseudo-tomentosa, while in A. tomentosa it is prominent above and below.
The hairy covering (fig. 277) in the leaf and axis of both the species of Aerua consists of a dense covering of candelabra hairs which are more numerous on the lower surface. The underground rhizomatous axis of A. tomentosa is quite glabrous. Clothing hairs in A. aspera occur in the form of uniseriate trichomes composed of a few basal cells and of a long terminal cell curved as on a hinge. Walls are characterised by warts by means of which the hairs dove- tail with one another and form a dense covering (fig. 282). Besides clothing hairs, there are external glands composed of a uniseriate stalk and of an ellipsoidal head (fig. 283). Uniseriate trichomes in P. lappacea (fig. 279) are smooth-walled and consist of a basal portion of a few short cells and of a terminal portion which is curved and is composed of longer and jointed cells. The clothing hairs are filled with air and become white ; they thus reflect light and glare and arrest transpiration.
Structure of the Axis. — The epidermis consists of small tabular cells with outer and inner walls thickened and convexly arched out- wards and inwards respectively. The primary cortex is characterised by collenchyma at the angles and by an assimilatory tissue which is composed of palisade cells in A. pseudo-tomentosa and of chlorenchyma in other members. In the rhizomatous axis of A. tomentosa the assimilatory tissue is not developed. The assimilatory tissue in P. lappacea, forms a continuous ring, while in other members it occurs between collenchyma groups at the angles. There is a layer of green cells resembling the cells of the bundle-sheath, close below the assimilatory tissue of both the species of Aerua.
There is extensive development of cork in the rhizomatous axis- of A. tomentosa. It extends between epidermis and pericycle and also between the latter and the groups of soft bast. It is partly of epidermal and partly of pericyclic origin.
