8 
in such a way that the new takes the place of the old, not only as an 
absorbing organ but in the mechanical system as well. At the point 
where dying back ceases, a root, or frequently two roots, spring from 
the end of the part which is still living. Their origin apparently is 
internal, and, as is to be expected, from the outer limit of the stele; 
the hypodermis of these older parts of old roots is so strong that the 
young ones are rarely able to break through it, with the result that they 
grow onward within the shell, sometimes for 30 centimeters or more, 
before the hypodermis is sufficiently decomposed to permit their escape. 
The direction ‘of growth is then well fixed. From an observation of 
exceptional cases in which the young root succeeded in rupturing the 
hypodermis at its origin, and in which it then grew along or near it, it 
appears that as a phenomenon of “correlation” the young root has the 
same orientation-reaction as the one it replaces. ‘The old hypodermal 
shell is a most effective aid in this reaction. 
My observations on the rapidity of the growth of roots have been un- 
satisfactory. Many times I have marked off zones on apparently healthy 
roots only to discover that they showed no subsequent growth. Some, 
for a time, have elongated little or not at all, then for a few days have 
grown vigorously, then stopped, without any apparent reason for the 
irregularity : 
The most rapid growth I have measured was 3.5 millimeters per diem. In a 
month three roots grew more than 4 centimeters, but none as much as 5 centi- 
meters. Sometimes, under favorable conditions, there may be a much more rapid 
growth than I have been able to observe; 3.5 millimeters per diem is hardly 
more than 1 meter per annum, a rate too slow to be accepted without more 
evidence. A part of the roots 1 examined grew in water and a part in air sur- 
rounded by soil. Those which elongated considerably in water at the same time 
became more slender. 
In large and rapidly growing roots a little elongation occurs in a zone 10 to 15 
millimeters from the tip (not from the growing point), but in most cases it is 
confined to the apical 10 millimeters. The root whose growth was most rapid 
was 9 millimeters in diameter and had a cap 10 to 11 millimeters long. In two 
days the latter grew 0.5 millimeter, 5 millimeters of root grew out of it, and the 
zone immediately outside grew 1.5 millimeters. The length of the cap is some- 
what greater than the diameter of the root, which is usually about equal to the 
length of the growing zone when measured from the outside tip; therefore all 
growth is generally within the cap. In this case the cap grew one-tenth as rapidly 
as the root, and this seems to be about the usual ratio. In the ground the resist- 
ance to the passage of the moving tip results in a continual tearing off of the 
outer layers of the cap, these layers usually persist in the form of collars around 
the root, and each is about as long as the cap; altogether they not infrequently 
form a sheath along the whole younger part of the root. It is possible that 
these collars or sheaths facilitate the absorption of water. When the root grows 
without friction, in water, the whole outer portion of the cap, while retaining 
its form, is occasionally sloughed off. 
No response to any other directive agent is so conspicuous as the autotropism 
of the coconut roots, of whatever order. The general level of the main roots is 
maintained, by a combination of hydrotropism and aérotropism, which I have not 
