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On the Unsymmetric Structure of the Leaves of Mnium spinosum 
In a pamphlet recently received M. Jacques Pottier 1 presents some inter- 
esting observations upon the structure of the costa of Mnium spinosum , a species 
that in Europe occupies something of the position here taken by M. spinulosum. 
Since some material collected for embryological purposes was not in satisfactory 
shape, the author tried to utilize it for morphological study. Being struck with 
the decidedly one-sided development of the leaf first examined, M. Pottier 
made a careful study of a few leaves to ascertain, if possible, the cause. Be- 
cause of the large amount of labor required, it was impossible to make detailed 
studies of more than two leaves, but similar conditions were observed in all 
leaves examined. Beginning at the apex, over 350 transverse sections were 
made from definitely ascertained regions of the leaf, proceeding in regular order 
toward the base; the sections were then drawn under a camera lucida and studied 
in detail with especial reference to the position and origin of the various cellular 
elements in the costa. 
The costa in this species is composed of several layers of cells, containing 
large cells (guides), smaller stereid cells, and a central strand of small parenchyma 
cells. These all seem to arise from repeated divisions of a single layer of rather 
large cells, new walls being formed both horizontally and perpendicularly. If 
the axis of morphological symmetry be taken as a line passing between the two 
middle guides of the central row and in the plane of their division wall, it is 
seen that this line is not perpendicular to the upper surface of the leaf, and that 
the greater mass both of the stereids and of the central strand usually lies either 
to the right or to the left of it. Serial sections show that this displacement alter- 
nates from right to left, with intermediate portions that are more nearly sym- 
metrical. Furthermore this one-sidedness is apparently associated with the 
sinuosities of the costa, being to the right when the nerve bends to the right of 
the median line and vice versa. The alternations of right and left asymmetry 
seem to be more numerous in the apical and extreme basal portions of the leaf 
than in the median part. 
M. Pottier suggests that the sinuosities of the nerve arise from the fact that 
the leaf is formed from a single apical cell cutting off new cells in two planes, 
that the resulting displacements of costal elements come from the compres- 
sions and expansions associated with this bending of the nerve, and that the 
greater abundance of the alternations of asymmetry in the upper and lower 
portions is connected with the fact that these regions are the ones where the 
least intercalary growth occurs in the elongation of the leaf. 
E. B. C. 
MISCELLANEOUS NOTES 
Dr. Alma G. Stokey, of Mt. Holyoke College, writes us that in the recent 
fire that destroyed the building in which the botanical department was located, 
1 Jacques Pottier. Sur la dissymetrie de structure de la feuille du Mnium spinosum (Voit) 
Schwaegr. pp. 1-16, with 28 figures in 7 unnumbered plates. Berne, Biichler & Co. 1917. 
