18 BORTHWICK—ADVENTITIOUS ROOTS AND RELATION 
or at the junction of a new shoot with that of the former year, 
but seldom in the middle of the shoot. He ascribes their origin to 
a growth-peculiarity in some trees in which the medullary rays 
have been unusually broad from the first, or become broad later, 
and project as wedge-shaped protrusions into the cortex which 
is ruptured. He states “that the majority of these projecting 
“medullary rays are covered by a woody-cone which is in 
“continuity with the last year-ring of the branch. Neither buds 
“nor leaf-rudiments can be recognised on these new formations, 
“so that they are to be regarded as wood-pegs, and the phenome- 
“non is to be explained as ‘ Kropfmaserbildungen.’” 
There is a strong resemblance between the Pyrus described 
by Sorauer and the one to be described in this paper. They 
seem indeed to be identical if one compares the illustrations and 
descriptions, although Sorauer’s interpretation differs from mine. 
The formation of adventitious roots on the stem is not un- 
common in willows and poplars, but in most cases some damage 
to the main root-system has preceded their production. 
According to Frank!, when the main root-system suffers 
injury, by any cause whatever (fungus, insects, physical con- 
ditions of soil), there occurs a production of new adventitious 
roots above the injured part, especially on the normally sub- 
terranean stem-portion of the perennating plants, but even also 
on the under part of the stem near the surface of the soil. 
Further, according to R. Hartig?, when the free access of air 
to the roots is prevented by too deep planting, the roots are 
killed. The tree either dies straight off, or may die off gradually, 
without being able to form new roots or replace the asphyxiated 
ones. Only a few trees, for example, willows, poplars, but more 
frequently shrubs, can develop numerous adventitious roots near 
the surface of the soil, and, like rootless slips, may form a new 
root-system. Similar conditions occur where earth is piled up 
round old trees, as often happens by the sides of roads and 
railway cuttings. 
That the willow can produce numerous adventitious roots even 
on fairly thick-barked trees may be seen in Plate VIIL., Fig. I. 
1Frank, Die Krankheiten der Pflanzen, 2nd Ed., Vol. I (1895), p. 91- 
*R, Hartig, Lehrbuch der Pflanzenkrankheiten, 3rd Ed., (1900), p. 265. 
