52 EXPERIMENT STATION. [Jan. 



ance. This breaking down of the tissne takes place in the more 

 advanced stages of disintegration and, according to Mr. Walker, 

 this broken-down mncilaginons substance exudes from the 

 cracks. Specimens of the bark and tissue showed this soft 

 layer, which v/as thicker directly under the cracks, where the 

 tension was less, to be anywhere from l/o to ll/o inches thick. 

 Microscopic examinations showed the fresh tissue to be com- 

 posed entirely of thin-walled parenchyma cells. 



In the illustration, 1 shows an enlarged cross section of a 

 piece of normal bark ; 2, a cross section of the bark near one 

 of the cracks ; 3, a cross section of bark and wood and the ab- 

 normal parenchymous growth ; and 4, a cross section of the nor- 

 mal bark, including some of the wood. In 2 and 3 the abnor- 

 mal growth is shown at e. In 3, which is an enlarged section, 

 the nature of the tissue is readily seen, as well as its origin 

 from the cambium /. In 5 is shown a diagrammatic cross sec- 

 tion of tree, with lesions. 



This peculiar cracking characteristic of Carolina poplars 

 poisoned by gas has never been observed by us in other species, 

 although like eifects have been noticed in willow cuttings grow- 

 ing in water charged with gas, in which case we have often no- 

 ticed a splitting of the bark and slight swelling, followed by the 

 formation of a mucilaginous mass under the bark. The bark 

 of most trees poisoned by illuminating gas remains intact until 

 it becomes very dry and brittle, and then it cracks, curls up 

 and eventually falls oif. There is, however, no disposition in 

 other trees, so far as noticed, for the tissue to become mucilagi- 

 nous. In some cases of poisoning of coniferous trees the bark 

 may become ruptured and a copious flow of pitch follow. This 

 tissue is apparently what is termed by Hartig " wound cam- 

 bium," and we have never seen it forming on poplars as the re- 

 sult of ordinary mechanical wounds. 



During the past season we have noticed a number of cracks 

 on small pojilars {Populus treniuloides) apparently resulting 

 from frost injury. But in this case there was no abnormal de- 

 velopment of the cambium.^ 



When banding substances are applied to Carolina poplars 



' Tho injection of poisons into tissue is said to cause a rupturing of the tissue owing to changes 

 in the turgescence of the cells. 



