514 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



in fruits, and Mangin (1888) found that plant-cells consisted of a combi- 

 nation of cellulose and pectose ; and that the latter constituted the 

 middle layer in many adult tissues. He regarded it as the first membrane 

 formed in cell division, or " the fundamental layer of the cellular mem- 

 brane." This seemed to indicate that he considered it as identical with 

 the " cell-plate of a dividing cell. 



In 1890 Mangin corrected his view, as he found the middle layer to- 

 consist, not of pectose proper, but of a cement of insoluble pectates. 



The intercellular substance forms a thin layer on the whole surface of 

 contact of adult cells ; when the cells separate, so as to form intercellular 

 spaces, these spaces are bounded by a pectic layer (detected by staining 

 with ruthenium red). 



With regard to its origin, Dippel said that cambial cell-walls are com- 

 posed of pectic acid, which, at least after the transformation of these 

 walls into the so-called " intercellular substance," exists largely in the 

 form of calcium pectate. 



As the cambial daughter-cells are transformed into bast and wood 

 tissue, the primary cell-walls, consisting of pectose and cellulose, are 

 deposited next to the now apparently simple cambial walls, which undergo 

 a transformation into the " intercellular substance." 



The combination of this latter -with the primary cell-wall forms (in 

 his \ iew) the middle lamella of mature tissues, against which secondary 

 thickenings are deposited. 



The author described his investigations upon Pinus sijlvestris, Nerium 

 Oleander, Rosa sp., Tilia americana, and Ilex opaca. He then criticises 

 the above view of Dippel. He finds that the middle layer is of a pectic 

 character, but that it undergoes changes with age. He thinks that 

 Dippel's idea that the intercellular substance is derived from the walls of 

 the cambium mother-cells has little evidence in its favour. 



The author prefers to regard this " intercellular substance " as repre- 

 senting pectic acid which has exuded through the cambium cell-walls into 

 an intercellular cleft, formed by the splitting of the radial wall. Its 

 ultimate fate is to be absorbed into the adjacent cell- walls. There is no 

 trace of this substance except in the Pine. 



Intercellular spaces in other plants can be accounted for only by the 

 rounding up and drawing apart at their corners of adjoining cells. This 

 induces a splitting of the middle lamella. 



Referring to the formation of the cell-plate in meristem tissues, the 

 author observes : " On the basis of the later investigations respecting the 

 relation of the cell-wall to the cell-plate, we have seen that the middle 

 layer appearing after the splitting of the cell-plate is to be considered as 

 formed by deposition from the split halves of the original plate ; the 

 middle lamella of mature tissues would include, then, in addition to 

 j)ossible later deposits, both" the layers dejDosited on the inner surfaces of 

 the daughter plasma membranes." 



The middle lamella is therefore of a double natm'e, capable of splitting 

 into two in forming intercellular spaces. 



Strasburger's view is confirmed that the cell-plate splits before the 

 new cell-wall is laid down, the latter thus having a double nature from 

 the start. 



