152 Parkin . — On some points in the 
becoming merismatic. Not finding in botanical literature 
any description of such an absciss-layer in these plants, it 
seems worth while making a note of it, and pointing out 
a few details connected with the occurrence. 
Von Mohl 1 in his researches on ‘ leaf-fail ’ in reference to 
Monocotyledons, mentions merely the falling of their perianth- 
segments and immature capsules. 
Bretfeld 2 has investigated the mode of detachment of 
leaves in Dracaena , many of the Orchidaceae and Aroideae, 
and generalizes from the study of these plants, that, whereas 
the leaf-fall in Dicotyledons is brought about by a new tissue 
formed a short time before the shedding of the foliage, in 
Monocotyledons it results from the action of a special 
mechanism, produced along with the other tissues in the 
developing leaf, similar to the contrivance which brings about 
the dehiscence of dry pericarps. However, the method here 
observed resembles that of Dicotyledons, although it is 
perhaps simpler in detail. 
Some time before the leaves turn yellow, certain of the 
parenchymatous cells situated a little way above the tunicate 
base of the foliage-leaf become merismatic, and divide to 
form a zone of narrow cells with conspicuous nuclei and 
abundant protoplasm ; this region is visible to the unaided 
eye as an opaque line on holding the leaf up to the light 
(Figs. 13, 14). In Galanthns nivalis the cell-divisions were 
just commencing when examined on April 1 6, and were well 
advanced on May 1 ; by the end of May the leaves, having 
turned completely yellow, are easily detached by means of 
their absciss-layers from their swollen bases, which now 
become the scales of the bulb, full of reserve material. The 
epidermal and mesophyll-cells and nucleated cells belonging 
to the vascular bundles take part in the divisions ; the 
raphide-cells and, of course, the vessels remain passive, their 
lumina becoming obliterated by the pressure exerted on them 
by the adjacent dividing cells. As a rule, about four or 
1 Mohl, Botanische Zeitung, i860. 
2 Bretfeld, Pringsheim’s Jahrbiicher, xii, 1880. 
