VARIOUS KINDS OF FOSSIL PLANTS 21 



the succeeding chapters constant reference will be 

 made to tissues preserved in "coal balls", and it may 

 be noticed that not only the relatively hard woody cells 

 are preserved but the very softest and youngest tissues 

 also appear equally unharmed by their long sojourn in 

 the rocks. 



Fig. 12. — Pliotogmph of Section tlirough a Bud ai Lepidodendron, sliowing many small 

 leaves tightly packed round the axis. From a ' ' coal ball " 



The particular value of the coal balls as records of 

 past vegetation lies in the fact that they are petri- 

 ifactions, not of individual plants alone, but of masses 

 of plant debris. Hence in one of these stony con- 

 cretions may lie twigs with leaves attached, bits of stems 

 with their fruits, and fine rootlets growing through the 

 mass. A careful study and comparison of these frag- 

 ments has led to the connection, piece by piece, of the 

 various parts of many plants. Such a specimen as that 



