228 



the first appearance of gum iu tLecell, tlie graius of starch, still entire^ 

 are gathered into little groups, around which appears athin layer of gum, 

 also small portions of which mnj be seen deposited in other jiartsof the 

 cell. The masses of starch enveloped by gum diminish continually 

 as the layer of gum increases in thickness, but when treated by iodine 

 the two substances preserve their special properties without modifica- 

 tion till the starch finally disappears, usually leading a small cavity in 

 the center of the little mass of gum. When the production of gum com- 

 mences in the tissues, an increased amount of starch is observable in the 

 neighboring cells which seems absorbed, and immediately changed into 

 gum, but ordinarily the gum in this case does not appear to be deposited 

 in the cells, but passes into the neighboring reservoirs where it accumu- 

 lat-es in considerable quantity. 



3. It is neither in vessels nor cells, but rather in the lacunar formed in 

 the interior of young tissues, the voluminous masses of gum accumulate, 

 which we often observe. These lacunffi are most frequently found in the 

 cambial zone, but may be seen at different depths in the wood, disposed 

 concentrically like successive annual layers. Tliey are formed in the ger- 

 minal layer, and then occupy the interval between the medullary rays. 

 When not too largely developed, a new woody layer forms ouside of 

 them, and the growth is not sensibly altered. On the contrary, if growth 

 cease at this i)oint, a flow of gum is caused, the woody tissue necroses 

 and cannot be covered except b}' the extension of lateral portions where 

 the germinal layer is uninjured. 



The tissues next to these lacun?e suffer an important modification 

 of development ; the cambium, instead of forming woody tissue, produces 

 cells in which an abundance of starch is deposited. There arises then, 

 wherever gum is developed, a particular tissue (wood}' parenchyma) 

 which does not exist in healthy stems, and whose appearance is so inti- 

 mately connected with the morbitl formation of gum, that it may be 

 consderedas a pathologic tissue. The starch, which accumulates in 

 this special wood}- parenchyma, is used, as in the medullary rays, to form 

 gum, which accumulates in large quantities iu the lacunre. These la- 

 cunte increase at the expense of the neighboring tissue, which is disor- 

 ganized ; nevertheless, the cells which border the lacuna? often manifest 

 extreme vital activity, and give birth to true pathologic formations. 

 They develop, multiply, and ramify in the interior of the lacunfe, even 

 when separated from the' rest of the tissue, and absolutely isolated in 

 the middle of the aiim. 



MICROSCOPIC OBSERVATIOiNS. 



By Thomas Taylor. 



J. M. Steel, of Harrisburgh, Ark., under date of August 10, 1873, 

 writes to the Commissioner of Agriculture as follows: 



I desire to make iuquiry in reference to tlie apple-speck, or rot. Our apples on Brow- 

 ley's Ridge are all serioiisly damaged this season by a speck or blister, of the size of a dime 

 or less, which apjjears on their skin. The speck becomes larger as the season advances, and 

 the apples at length decay on the trees and fall oiT. This speck has been observed on our 

 apples for some years past, but has never been so destructive to the fruit as during the last 

 season. The fruit begins to rot when it has attained its full size, apparently from a defi- 

 ciency of proper food necessary for maturing it. We sometimes think that the rot is occa- 

 sioned by the sting of a curculio, but as we have no means of searching out the cause, we 



