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POPULAR SCIENCE REVIEW. 
nearly the ultimate position of the cc separating layer,” yet it is 
not itself to be considered as the commencement of that layer. 
The constriction may readily be seen throughout the whole 
summer, and by microscopical examination it may easily be 
noticed to be purely superficial; moreover, it is much more 
marked on the lower than on the upper surface ; while the 
“ separating layer,” on the other hand, travels, as we have seen, 
from within or from above downwards and outwards. The 
reader will find it easier at any time to detach a leaf from the 
stem by bending it downwards than by forcing it upwards, a 
result that would be reversed if the constriction were really, as 
it is so often considered to be, the provision made aforetime for 
the ultimate removal of the leaf. What, then, is this constriction ? 
To answer this question, we must refer to Eichler’s thesis on the 
development of leaves, of which Dr. McNab * has given an 
abstract in English with comments and additional observations 
of his own. According to these observations, the first stage in 
the formation of the leaf is the production of a little pimple of 
cellular tissue on the side of the stem ; this receives the name of 
<f phylloblast,” and after a time it divides into two portions, a 
proximal portion next the stem, which Dr. McNab calls the 
6C hypophyll,” and a distal portion called the te epiphyll;” from 
this latter, the true leaf with its stalk is evolved, from the 
former (which seldom assumes any large size) the stipules are 
produced when these organs are developed. 
Applying these facts to the fall of the leaf, it will be seen 
that the separation takes place below the swollen base of the 
leaf-stalk, beneath what is technically called the u pulvinus,” and 
hence the constriction so often seen in that situation indicates 
the original line of separation between the “ hypophyll ” and the 
“ epiphyll.” This at any rate seems ver } 7 probable. But whether 
this be so or not, the detachment takes place, not precisely 
between leaf and stem, but between leaf and <£ hypophyll,” the 
latter being intermediate in position between leaf and stem, and 
structurally more nearly allied to the latter than to the former. 
The formation of a separating layer in the way we have explained 
is not unique ; analogous instances may be seen in the fall of 
the fruit. On the freshly detached surface of a pear-stalk, a 
white mealy appearance may be seen by the naked eye ; on 
microscopical examination this will be found to be due to the 
formation of a layer of new cellular tissue. 
On the whole, then, of all the assigned causes for the fall of 
the leaf, this last, dependent on an alteration, or rather on a new 
growth in the leaf itself, is the most important, and probably the 
only one of itself sufficient to produce the result. It remains now 
* Trans. Bot. Edinb. 1865, 1866. 
