So ANCIENT PLANTS 



which tell us so much about the more ancient plants. 

 Cases are known of more or less isolated fragments with 

 their microscopical tissues mineralized. For example, 

 there are some palms and ferns from South America 

 which show their anatomical structure very clearly pre- 

 served in silica, and which seem to resemble closely the 

 living species of their genera. The bulk of the plants 

 preserved from these periods are found in the form ot 

 casts or impressions (see p. lo), which, as has been 

 pointed out already, are much less satisfactory to deal 

 with, and give much less reliable results than specimens 

 which have also their internal structure petrified. The 

 quantity of material, however, is great, and impressions 

 of single leaves innumerable, and of specimens of leaves 

 attached to stems, and even of flowers and fruits, are to 

 be found in the later beds of rock. These are generally 

 clearly recognizable as belonging to one or other of 

 the living families of flowering plants. Leaf impres- 

 sions are by far the most frequent, and our knowledge 

 of the Tertiary flora is principally derived from a study 

 of them. Their outline and their veins are generally 

 preserved, often also their petioles and some indication 

 of the thickness and character of the fleshy part of the 

 leaf From the outline and veins alone an expert is 

 generally able to determine the species to which the 

 plant belongs, though it is not always quite safe to 

 trust to these determinations or to draw wide-reaching 

 conclusions from them. 



In fig. 59 is shown a photograph of the impression 

 of a Tertiary leaf, which illustrates the condition of an 

 average good specimen from rocks of the period. Its 

 shape and the character of the veins are sufficient to 

 mark it out immediately as belonging to the Dicotyle- 

 donous group of the flowering plants. 



Seeds and fruits are also to be found; and in some 

 very finely preserved specimens from Japan stamens 

 from a flower and delicate seeds are seen clearly im- 

 pressed on the light stone. In fig. 60 is illustrated a 



