VOLUME XXXII NUMBER 1 
DOTANICAL (aA ZETEE 
JULY, roor 
ON THE ORIGIN AND NATURE OF THE MIDDLE 
LAMELLA. 
CHARLES BE. ALLEN: 
THERE has long been recognized in the cell walls of plant 
tissues, and particularly in the thickened walls of bast and xylem 
elements in woody plants, a central layer or plate, sharply dis- 
tinguishable by its optical and staining properties from other 
regions of the wall. To account for the origin of this layer, 
apparently homogeneous and equally closely related to each of 
the cells between which it occurs, various theories have been 
advanced, and the names “primary cell wall,’”’ “intercellular 
Substance,” and ‘‘ middle lamella” indicate the diversity of opin- 
ion that has prevailed as to its real nature. 
The theories of the origin of this ‘‘ middle lamella,” adopting 
as most convenient its commonest designation, may (1), generally 
Speaking, be grouped under three heads: (1) those which hold 
it to have been originally a common matrix in which the cells 
were imbedded; (2) the theory that the middle lamella is the 
Original cell wall, laid down in common by the two daughter cells 
in the process of cell division, and remaining distinct from the 
secondary thickening layers deposited upon its either face ; and 
(3) the view that it is a substance excreted by the daughter cells 
into a space left between them after their formation, or into a 
Space formed by their rounding up and drawing apart. 
The first theory, that the middle lamella is an original com- 
mon matrix of the cells, was shown to be groundless as soon as 
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