84 



THE PRESERVATION OF PLANTS AS FOSSILS. 



[CH. 



solution assume an appearance almost identical with that of 

 stained sections of recent plants. The spaces left by the 

 decayed walls act as fine capillaries and suck up the coloured 

 solution \ 



In the Coal-Measure sandstones of England large pieces of 

 woody stems are occasionally met with in 

 which the mineralisation has been incom- ji 



plete. A brown piece of fossil stem lying / 



in a bed of sandstone shows on the |.l 



surface a distinct woody texture, and the q 



lines of wood elements are clearly visible. f 1 



The whole is, however, very friable and J^j 



falls to pieces if an attempt is made to 

 cut thin sections of it ; the tracheids of 

 the wood easily fall apart owing to the 

 walls being imperfectly preserved, and 

 the absence of a connecting framework 

 such as would have been formed had the 

 membranes been thoroughly silicified. 

 It is occasionally possible to obtain from 

 petrified plant stems perfect casts in 

 silica or other substances of the cavity 

 of a sclerenchymatous fibre, in which the 

 mineral has been deposited not only in 

 the cavity but in the fine pit-canals 

 traversing the lignified walls. Such a cast 

 is represented in fig. 16, the fine lateral 

 projections are the delicate casts of the 

 pit canals. Numerous instances of minute 

 and delicate tissues preserved in silica are 

 recorded in later chapters. A somewhat 



unusual type of silicification is met with in some of the 

 Gondwana rocks of India, in which cycadean fronds occur as 

 white porcellaneous specimens showing a certain amount of 

 internal structure in a siliceous matrix. Specimens of such 

 leaves may be seen in the British Museum. 



1 I am indebted to Dr Renault of Paris for showing to me several preparations 

 illustrating this method of petrifaction. 



Fig. 16. Internal cast 

 of a sclerenchymatous 

 cell from the root of a 

 Cretaceous fern (Rhi' 

 zodendron oppoliense 

 Gopp.). After Stenzel 

 (86) PI. III. fig. 29. 

 X 240 and reduced to 

 one-half. 



