THE 
NEW PHYTOItOGIST. 
Vol. XI, No. 4 . 
April, 1912 . 
[Published April 29th, 1912]. 
ON SOME METHODS IN PALEOBOTANY. 
Bv H. Hamshaw Thomas. 
)HE remains of the plants of past ages have generally been pre¬ 
served in two ways. The first is by petrifaction, when the 
original cell outlines are preserved by the infiltration and solidifi¬ 
cation of lime, silica or some similar substance. The second method 
is by the formation of an impression or cast in a sandy or muddy 
matrix, the original plant-tissue being reduced to a coaly layer. 
But we have also in some cases a rather different form of preservation 
from the last mentioned ; here the plant has been embedded in mud, 
little decay has taken place, and the remains which are now found 
are best described as “ mummified plants.” In the case of leaves, the 
cuticles of both surfaces often remain, sometimes with portions of 
the tougher cells, the cellulose has become more or less converted 
into carbon, and we have here a stage in the production of the 
ordinary type of impression in which all the plant-tissue is reduced 
to a thin coaly layer. 
From the point of view of the botanist, of course, the petrified 
plants are by far the most important. It has, however, rarely 
happened that the conditions were suitable for the petrifaction of 
plants, and even when we have petrified remains they are often very 
fragmentary. Impressions are far commoner, and though we do 
not gain so much useful knowledge from them, they often exhibit 
the external features of plants with great clearness; they aid us in 
piecing together scattered fragments of the various organs of a plant, 
and sometimes furnish useful information as to the form of the 
reproductive organs. 
The mistakes of some of the earlier-and ever of the later-palaeo- 
botanists, whose observations were confined to plant impressions, 
