146 MASSACHUSETTS HORTICULTURAL SOCIETY. 



It is a well-known fact that where the cambium is exposed in 

 wonucls, it produces new cells more vigorously than the cambium 

 of uninjured stems. The reason for this, at first sight, anomalous 

 state of things Avill be easily understood if we call to mind the 

 tension of any normal trunk. A trunk may be regarded as a 

 cylinder composed of a solid axis of Avood whose circumference is 

 formed of the actively growing cambium encircled by the inner 

 and outer bark, which taken together we may now, for con- 

 venience's sake, call the cortex. The different parts of this 

 compound cylinder grow under different tensions. On the one 

 hand, the inner parts, as they grow, exert a strong outward 

 pressure on the cortex, while, on the other hand, the cortex acts 

 as a sheath which exerts a strong pressure on the parts within. 

 That when the normal pressure is interfered with, the relative 

 growth of the different parts of the stem is changed, is well sliown 

 if a slit is made through the cortex to the region of the cambium. 

 The cells of the cambium thus freed from pressure from without 

 grow more rapidly than before in the direction of the slit, so that 

 the wound thus made is rapidl}^ filled by the new cells thus formed, 

 and the new growth may even be so great as to more than till the 

 gap, and cause a slight protuberance on the wounded side. 

 Furthermore, when the tension of the cambium is relieved by the 

 removal of the cortex, its function of producing new wood cells 

 on its inner side is altered, and microscopic examination of the 

 new wood formed in wounds shows that the wood cells are shorter, 

 and the vessels decidedly less numerous than in normal wood. 



The description which I have given of the wa}' in which the 

 callus arises, although you may perhaps think it somewhat compli- 

 cated for a popular lecture like the present, is, in realit}', a brief 

 attempt to sketch the process in its main points onl}^ omitting 

 many details which are of interest to specialists. AYhat I have 

 described is the normal mode in which the healing process begins, 

 and is to be seen in those seasons of the year when the cambium 

 cells are active. During the colder months of the year, however, 

 the cambium is in a dormant condition, and if wounds occur at 

 such seasons, the cambium is not able to form a callus at once, 

 and the process just described does not begin until the season of 

 plant growth returns. Meanwliile the exposed parts will probably 

 have been more or less affected by weathering, and the closing of 

 the wound by natural processes is made more difficult. In the 



