RUBBER (HEVEA) DISEASES 69 
dormant or imperfectly developed and are known as preventitious 
buds, for the reason that they occur on the stem from the beginning. 
These buds are provided with their own cambium and increase in 
length only so far as to keep up with the annual increment of the 
parent stem. They extend their growth beyond only when the 
parts above are injured. It is these buds which give rise to shoots 
from the sides of stumps or which replace the branches that have 
been killed by Dothidella or other agents. 
The mutilation of the trunk by excessive tapping, pruning, and 
defoliation by insects or fungi may also initiate the development 
of these buds into shoots, the requirement being that they receive 
an excess of nourishment. Not infrequently these buds through a 
cessation of growth in their own cambium become disconnected 
from the wood of the parent stem and by a later concentric disposi- 
tion of wood give rise to spherical or oval nodules embedded in the 
cortex (PI. XXXII, A). They may by exfoliation of the bark be 
gradually brought to the surface, and since they have no connec- 
tion in the late stages with the wood of the tree they may be easily 
detached or may eventually fall with the bark scales. These globu- 
lar shoots or sphaeroblasts, as they have been called, rarely exceed 
2 inches in diameter. On some coniferous trees they may attain 
a diameter of 9 inches or more. Smaller ones are common on beech 
and other dicotyledonous trees. These nodules under the influence 
of various stimuli may develop into shoots. They may occur on 
the smooth bark at any point on the main stem, but are more com- 
mon at the root collar and in the region of old leaf scars. 
Preventitious buds may be distinguished from cortex nodules by 
their structure and mode of origin. 
Under the influence of certain stimuli bud structures may origi- 
nate on fully developed parts of the tree, becoming what are known 
as adventitious buds. If their development is arrested they form 
elongated structures with concentric arrangement of the wood. They 
usually originate on the callus of such a wound as may occur at the 
top of a stump or on tapping wounds and are attached usually by a 
broad base (PI. XXXII, B). It is these buds which give rise to cop- 
pice on the stump and roots when the tree is cut or killed above 
ground. In this case they may originate endogenously on the un- 
injured cortex, or they may develop from the meristematic tissues 
newly formed at the edge of a wound. New shoots originating from 
these buds are commonly found on the stems of young trees which 
have been cut down. On a few of the stumps shoots originating 
from both preventitious and adventitious bud structures have been 
observed. Owing to the rapidity with which the exposed wood be- 
comes infected with wood-destroying fungi, Hevea stumps produce 
very few new shoots. If such shoots attain tapping size the heart- 
wood at the base is liable to become infected from the diseased 
wood of the parent stump. 
Steinmann reports the occurrence of globular aggregations of un- 
developed buds on Hevea in Java. He states that such abnormal 
development may be attributed to the stimulus of an intracellular 
fungus, the mycelium of wdiich is found principally in the tissues 
of the pith. It is not stated whether the buds are of primary or sec- 
ondary origin, 
