318 SIGNOR U. MARTELLI ON THE CAUSE OF 
The tissues of the corolla are formed, in the lower part, of ovate 
or oblong, large, rather irregularly shaped cells, which contain a 
few small grains of starch (fig. 1,@). The cells gradually become 
narrower and longer, and the presence of the starch augments in 
the cells of the short tube of the corolla. The thalamus is also 
composed of large round cells, though they are different in shape 
from those of the corolla (fig. 1,7). But where the corolla is 
inserted in the thalamus it is easy to observe a zone of a few rows 
of opaque, slightly yellow-coloured, and minutely granular cells 
(fig. 1, 6). These rows of cells constitute a stratum which dis- 
tinetly divides the tissue of the thalamus from the overlying one 
of the corolla. This stratum forms the separating zone, by the 
aid of which the corolla easily detaches itself from the thalamus. 
Also in the leaves an identical structure is found, which in like 
manner brings about their fall *. 
In the fully expanded flower the tissue of the corolla and of 
the thalamus which is nearest to the separating zone, when treated 
with chloriodide of zine, turns blue, while the separating zone 
assumes a more or less yellow tint, scarcely visible towards the 
* Mohl, H., “ Ueber die anatomischen Veriinderungen des Blattgelenkes, 
welche das Abfallen der Blitter herbeifiihren,” Botanische Zeitung, xviii. pp- 1-9 
(1860). Van Tieghem et L. Guignard, ‘Observations sur le Mécanisme de la 
Chute des Feuilles,” Bull. Soc. Bot. France, 1882, p- 312. 
