200 Reserve Material of Plants. 



decrease in size, the chlorophyll undergoes modifications or is 

 imperfectly formed, an abnormal red, yellow and green color is 

 developed, and all these conditions continue to increase until the 

 extremes are reached. 



The last external characteristic is to be found in the abnormal 

 development of the new wood. The branches of the new growth 

 become more strongly atrophied as the disease advances, until they 

 finally become of a very wiry character and develop upon the 

 trunk and branches in clusters. 



The internal features are as strongly marked as the external, 

 and may generally be determined in very early stages of the dis- 

 ease. The first indication is to be found in the very dense accu- 

 mulation of starch, not only in the pith and medullary rays, but 

 particularly in the bark, from which it should normally be absent 

 to a very large degree. This excessive accumulation of digested 

 material in unusual parts, is at once indicative of an imperfect 

 power of distribution to the growing parts and inability of the 

 plant to convert it into tissues, so that the atrophy of structure 

 appears in the first instance, not to be caused by want of material* 

 but by the absence of certain chemical compounds by which the 

 necessary chemical changes of direct nutrition may be accom- 

 plished. 



This accumulation of starch increases as the disease progresses, 

 while, at the same time, very important modifications in the tissues 

 themselves, are developed, particularly in the bark. There the 

 cells of the middle bark, or mesophlceum, become relatively thick- 

 walled ; the intercellular spaces decrease in size and number and 

 thereby retard the proper respiratory function ; the disposition of 

 the cells becomes somewhat regular, the tendency being to the 

 development of layers forming well-defined concentric rings, 

 while the form also tends strongly to an elongated ellipse with its 

 minor axis running in a radial direction. 



Contrasting this with the normal, we find in the latter that both 

 the internal and external features are markedly different. The 

 leaves are a deep green, and of large size, as already shown. The 

 young shoots, likewise, are of a lively green color, and two or 

 three times the diameter and several times the length of the dis- 

 eased. Internally, the starch, in comparatively small quantity, is 

 confined almost wholly to the pith rays and wood, the bark con- 



