Physiological Studies in Plant Anatomy 261 
standing them in water or applying it under increased or reduced 
pressure by means of a mercury column. The pressures were small, 
averaging about ten centimetres of mercury. The differences were 
very striking; under even this small reduced pressure these leaves 
were soon drooping and dry and even the young leaves ceased to 
grow. These leaves had to be left out of the comparison, but the 
leaves under increased pressure compared with those standing in 
water showed striking differences within the short period April 29th 
to May 12th. Under pressure a sharp brown edge had formed around 
the cut in one day, whilst without the pressure the brown edge 
formed much more slowly. By May 12th the cuts in the leaves under 
pressure could be distinguished with the naked eye by the thin 
translucent line, due to the formation of a meristem bordering the 
clearly defined brown margin on all sides. In the leaves not under 
pressure the brown margin was wider and lighter and there was still 
no meristem. In young leaves under pressure meristem formation 
was clearly visible in sections by May 10th. The interesting exfolia¬ 
tion patterns described by Blackman and Matthaei in the leaves of 
Prunus Laurocerasus kept in a moist atmosphere, seem to receive 
considerable elucidation if it is considered that the formation of the 
meristem, functioning here as an absciss layer, depends upon a sap 
pressure, the incidence of which throughout the leaf is determined 
by the venation system. 
II. Leaf Fall 
The formation of periderm at the leaf scar requires only brief 
consideration for which the necessary data are provided by 
Tison’s ( 28 ) very comprehensive memoir on the subject. Reference to 
this paper will show that at the time the leaf falls, as the result of 
various types of activity in an absciss layer, the cushion of cells at 
the surface of the scar has already undergone, or immediately under¬ 
goes, a process of lignification and suberisation, the lignification being 
specially strong in the deeper seated layers. When the tissues of the 
leaf scar are blocked in this manner, the vascular strands being 
blocked by tylosis or the deposit of wound gum or other substance 
within the vessels and tracheids, then the formation of periderm 
may occur within the cushion, below the suberised and lignified 
surface layers. In no case does the meristem arise before the tissues 
of the scar are sealed by the previous suberisation. On the contrary 
in many cases no phellogen appears until the sap rises in the following 
spring; in the relatively few cases where the meristem is formed in 
