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Periods. The specimens in this case complete the sequence 
of plant life on the earth and bring it up to modern times. 
A number of specimens at one end of the case show the 
methods of preservation by petrifaction, incrustation and 
carbonization and on the upper shelf is a series of speci- 
mens from Quaternary and more recent swamp deposits which 
show how the conversion of living plants into fossils, a 
process now going on, has its beginning. 
The specimens in the adjoining wall-case further illustrate 
the characteristics of the plants of the late geological periods 
and the methods by which the various plant structures have 
been preserved. 
A number of specimens of silicified woods show the method 
of preservation by what is known as petrifaction, or conversion 
into stone, in which the woody structure is replaced by mineral 
matter. Other specimens show preservation by incrustation, 
in which mosses and the stems of reeds are coated or incrusted 
by mineral matter deposited from springs; while on the upper 
shelf and on the top of the case are logs and stumps from old 
swamps and interglacial deposits, in which the wood has been 
partially carbonized, or converted into lignite, by the slow 
process of natural distillation. This process represents the 
beginning of the conversion of vegetable tissue into coal. 
LECTURES 
Other features of the museum building include the large 
public lecture hall, with a seating capacity of over seven 
hundred, which occupies the western end of the basement. It 
is equipped with an electric projection-lantern, and public 
popular lectures covering a wide field of botanical and horti- 
cultural subjects are delivered here on Saturday afternoons 
in autumn and spring; these are fully illustrated by means of 
a very extensive collection of lantern slides owned by the 
Garden which is constantly being increased; a noteworthy 
part of this collection is the series of delicately and accurately 
colored slides of flowers, fruits, trees and shrubs, by Mrs. 
