RELATION OF STRUCTURE TO FUNCTION IN ROOTS. 753 



entrance into water-pipes, grow so extensively that in a shoi-t time the pipes are 

 entirely blocked up and the water-flow in them interrupted. The net-works of roots 

 taken out of these pipes resemble plexuses of hair, so abundantly are their tresses 

 developed. Hyacinths and many other bulbous plants, and even various foliage- 

 trees, as, for example, maples and horse-chestnuts, whose roots usually grow in the 

 ground, can be reared with the best results if their roots are allowed to grow 

 in water, provided that the water contains the necessary food-salts in adequate 

 amounts. 



Aerial roots (radices aerece) are developed on the periphery of the erect caudices 

 of tree-ferns, and in great profusion on the stems of epiphytes, especially of aroids 

 and orchids. In species of the tree-ferns Todea and Dicksonia the aerial roots are 

 all very short but so numerous and crowded together that they form an actual 

 mantle roimd the eaudex. In orchids growing on the bark of old trees the aerial 

 roots also arise in great numbers close together, but are always elongated and 

 filamentous, and form manes, as shown, for example, in Oncidium, figured on 

 p. 221. In otiier orchids, however, they may occur singlj^, and are then usually 

 much thicker, fairly stiff, and curved sinuously in and out or spirally twisted as in 

 the Sarcanthus rostratus, illustrated on p. 107. As already stated, in many aroids 

 and orchids they appear arranged with great regularity, either singly or in pairs, 

 opposite the points of insertion of the leaves on the stem. All these aerial roots 

 are excellently adapted by their structure (described on p. 222) not only for the 

 absorption of water and solutions of food-materials, but also for the condensation 

 of aqueous vapour from the surrounding air. In most instances they are enveloped 

 by a papery covering; more rarely they are thickly beset with so-called root-hairs, 

 and then have a velvety appearance. Most of those with root-hairs are rusty 

 brown in colour, whilst the others are white in dry air, and greenish in wet weather 

 • — in consequence of the abundant chlorophyll contained in the tissue below their 

 papery envelope. 



We must carefully distinguish these condensing aerial roots from such as, whilst 

 springing from epigeal stems and surrounded by air, are unable to condense aqueous 

 vapour or to absorb atmospheric water. These, on the other hand, grow down to 

 the ground and must penetrate into it in order to obtain the water and food-salts 

 they require. These root-structures are especially observed in climbing plants in 

 which the lowest internodes have died, and which then no longer stand in direct 

 connection with the soil. Their large foliage-leaves nevertheless require a much 

 greater amount of water than could be obtained from the tree-trunks on which 

 they support themselves. The large-leaved aroids illustrated on p. 36.5, whose 

 cord -like roots, from 4 to 6 metres in length, descend to the ground, may be 

 regarded as typical of this class. These forms are indeed called aerial roots, but 

 if we adhere to the distinction given above, they would be more accurately regarded 

 as a special class of subterranean roots. But since it has been repeatedly observed 

 that the aerial roots of some orchids, when they come in contact with the ground, 

 penetrate into it and assume the chai-acter of subterranean roots, the boundary 



Vol. 1 48 



